How to Choose the Right Builder in London: 12 Checks Before You Sign a Contract

Choosing a builder is one of the most important decisions in a renovation, extension or refurbishment. The right contractor must do more than carry out construction work: they need to understand the information, coordinate people and trades, manage risk, communicate clearly and bring the project to completion.

A lower quotation may be perfectly genuine. It may also cover a different scope, rely on optimistic allowances or collect too much of the contract value during the early stages. The only useful comparison is therefore a comparison of the company, the scope, the price allocation, the payment terms and the proposed delivery team.

This guide combines current UK consumer and company checks with practical lessons from Alt Construction's experience of pricing London projects and being asked to review or complete work after another appointment has gone wrong.

Quick Answer

Before appointing a builder, confirm who you are contracting with, whether they have relevant experience, what is included in the quotation, how the contract value is distributed and when payments become due. Also check the delivery team, programme, insurance, contract, variation procedure and communication arrangements.

The most useful question is: after each proposed payment, would the unpaid balance still be enough for a competent contractor to complete the remaining work?

Check	What useful evidence looks like
1. Business identity	Legal or trading name, address, company details and VAT information match the quotation and contract.
2. Relevant experience	Recent work comparable in scale, structure, logistics and level of finish.
3. Projects and references	Genuine completed projects, recent clients and evidence of the construction process.
4. Design readiness	The builder has reviewed the available drawings, specifications and technical gaps.
5. Like-for-like scope	Inclusions, exclusions, assumptions, allowances and VAT can be compared properly.
6. Price allocation	Early-stage values are credible and sufficient money remains for later work.
7. Delivery plan	A realistic programme, named management responsibility and adequate resources.
8. Insurance and safety	Current cover, relevant activities, suitable limits and clear dutyholder arrangements.
9. Payments	Deposits and valuations are defined, measurable and linked to genuine progress.
10. Written contract	The parties, documents, price, time, payments, changes and completion are recorded.
11. Variations and uncertainty	A written approval process and a sensible plan for concealed conditions.
12. Communication	Clear answers, written follow-up and realistic commitments before work starts.

1. Confirm Who You Are Actually Hiring

Start by identifying the exact person or business that will enter the contract. The name on the quotation, contract, invoice, insurance documents and payment instructions should make sense together. If different names or bank details are used, ask for a clear explanation before paying anything.

For a limited company, the free Companies House service can show its status, registered office, directors, filing history, previous names, accounts and registered charges. Identity verification introduced from November 2025 is improving confidence in who is behind companies, but the register still does not certify construction quality, competence or financial stability.

If VAT is being charged, HMRC's online service can confirm whether the VAT number is valid and the business name and address to which it is registered. A sole trader will not necessarily appear as a limited company, but the homeowner should still obtain the trader's full name, trading address and contact details.

Practical point: a professional website, branded vehicle or trading name is useful marketing, but the contract must still identify the legal person or company responsible for the work.

Where unrelated businesses use the same or a similar trading name, verify the company number, official website, telephone number, email domain and service area rather than relying on the trading name alone.

How to verify Alt Construction: Alt Construction is the trading name of ALT GROUP CONSTRUCTION LTD, registered in England and Wales under company number 11756956. Our official website is www.altconstruction.co.uk, and our quotations, contracts and invoices identify the same legal company.

2. Look for Experience Relevant to Your Project

Not every competent builder is suitable for every project. Decorating and maintenance work require different systems from a structural extension, and a new-build site is different from altering an occupied Victorian or Edwardian house with restricted London access.

• Relevant experience may include:

• structural openings, steelwork and temporary support;

• working around existing buildings and neighbouring properties;

• drainage, public sewers and difficult foundations;

• coordination of old and new construction;

• occupied-property protection, access, storage and deliveries;

• management of several trades, inspections and long-lead products.

Ask for examples similar in scale and complexity, not only photographs of attractive kitchens or bathrooms. Also ask what role the company actually performed: main contractor, subcontractor, labour supplier or project manager.

3. Review Completed Work and Speak to Previous Clients

Finished photographs are useful, but they show only part of the project. Where possible, look for evidence of structural stages, service coordination, protection, making good and the final finish. Recent projects are generally more informative than a portfolio based mainly on older work.

• When speaking to a previous client, ask:

• Was the quotation clear about inclusions and exclusions?

• Who managed the site and how regularly were updates provided?

• Were additional costs explained and agreed before work proceeded?

• How did the contractor deal with problems or changes?

• Did the company return for agreed snagging and completion items?

• Would the client appoint the same company again?

Online reviews can help identify patterns, particularly recent comments about communication, variations and completion. They should still be considered alongside direct references, company checks and actual project evidence rather than used as the only test.

4. Check Whether the Project Is Ready to Price

A reliable quotation depends on reliable information. For a substantial project, the contractor may need architectural drawings, structural information, Building Regulations details, specifications, schedules and known service or drainage requirements.

The builder does not replace the appointed architect or engineer. However, a contractor pricing complex work should be able to identify obvious gaps, contradictions and decisions that materially affect the construction cost. Planning drawings alone may define the size and appearance of an extension without explaining how every junction, structure or service will be built.

If important information is missing, each contractor will fill the gaps with different assumptions. The resulting figures may look comparable while representing different projects. It is often better to resolve the major design questions before seeking a final construction quotation.

5. Compare the Scope, Not Just the Headline Price

A quotation of £145,000 is not necessarily cheaper than one of £158,000 if the first excludes work that the second includes. Before ranking the prices, create a like-for-like comparison of the main inclusions, exclusions and allowances.

Common difference	Why it changes the comparison
VAT	A figure excluding VAT cannot be compared directly with a VAT-inclusive total.
Preliminaries and London logistics	Site management, protection, scaffolding, parking, permits, welfare, deliveries and waste may sit in different sections.
Structure and temporary works	Steel installation, bearings, propping and temporary support may be included, provisional or excluded.
Services and finishes	Electrical, plumbing, heating, flooring, decoration, kitchens and sanitaryware may be supply-and-fit, installation only or client supplied.
Professional and statutory costs	Design, engineering, Building Control, Party Wall and water-company costs may be outside the contractor's price.
Allowances and assumptions	A provisional amount is not the same as a firm price for a defined product or scope.

A quotation does not need hundreds of lines, but it should contain enough detail to understand the proposed work, the basis of the price and which costs remain with the homeowner.

6. Check Where the Money Sits - Not Only the Total

One warning sign we sometimes find in a lower quotation is that it is heavily front-loaded. A disproportionate amount of the contract value is placed in early items such as the deposit, preliminaries, strip-out, demolition, groundworks or structural work, while much smaller amounts are left for making good, plastering, second fix, joinery, flooring, decoration, testing, snagging and handover.

Early stages can genuinely be expensive. Structural steel, scaffolding, waste, site setup and specialist orders may require significant expenditure. Front-loading is therefore not proof that a contractor is dishonest. It becomes a concern when the allocation does not appear to reflect the real value and cost of the work.

On projects paid through valuations, a work item shown as 100% complete may be claimed or certified at the full amount allocated to it, depending on the contract. If that item was substantially overstated, the client can pay more than the reasonable value of the progress achieved. The contractor becomes well paid early, while too little value remains for the labour, materials and management needed to complete the later stages.

This is often when pressure begins: progress slows, requests for extras increase, finishing work is rushed or the contractor loses interest in completing the project. If another company must take over, the homeowner may have to fund part of the work again.

A simple financial test: after every proposed valuation, compare the unpaid balance with the realistic cost of completing everything still outstanding. If the balance would not be enough, ask the contractor to explain and rebalance the schedule before signing.

7. Ask Who Will Deliver the Work and How

A strong portfolio does not automatically tell you who will run your project. Ask who will be the day-to-day site manager, who can approve decisions, which trades are expected and how the contractor will coordinate subcontractors or specialist suppliers.

A useful pre-construction discussion should cover:

• the proposed start date and realistic duration;

• the sequence and principal dependencies;

• site supervision and frequency of management visits;

• working hours, access, parking, storage and waste;

• whether the property will remain occupied;

• long-lead items and who is responsible for ordering them;

• how progress and decisions will be reported.

Programmes change when design decisions, approvals, client selections or concealed conditions change. The important sign is not an unrealistic promise that nothing will move; it is a clear baseline, identified dependencies and a sensible process for updating the client.

8. Check Insurance, Safety and Responsibility

Ask for current insurance evidence in the name of the contractor you are appointing. Check the policy period, activities covered, limits, important exclusions and whether the arrangements are suitable for the proposed work.

• Depending on the business and project, relevant cover can include:

• public liability insurance;

• employers' liability insurance where legally required;

• contract works or contractors' all-risks cover;

• professional indemnity where the contractor accepts defined design responsibility.

The homeowner should also notify their own building insurer before substantial work begins. The existing house, the new work, temporary works, client-supplied products and any occupied areas need a coherent insurance arrangement; a public-liability certificate alone does not answer every question.

Construction duties should also be clear. Under CDM 2015, contractors must have the skills, knowledge, experience and organisational capability to carry out work safely. Where more than one contractor is involved, a Principal Designer and Principal Contractor must be appointed in writing, with domestic-client duties normally passing as the Regulations provide.

For regulated specialist work, check the relevant competence and certification route - for example, Gas Safe registration for gas work and suitable electrical certification at completion.

9. Understand the Deposit, Valuations and Final Balance

A deposit or initial payment can have a legitimate purpose, such as reserving a start date, mobilisation or agreed procurement. The amount, purpose and treatment if the project is delayed or cancelled should be written down. Paying the full contract value before work starts removes important protection.

For longer projects, payments may be made at stages or through periodic valuations. The contract should explain who assesses the work, how much can be claimed, the due dates and what evidence is needed for materials not yet incorporated into the property. If the client is paying for off-site materials, ownership, identification, storage and insurance should be addressed rather than assumed.

On larger projects, an architect, contract administrator or quantity surveyor may assess valuations. Where the homeowner administers payments directly, the quotation and payment schedule need to be especially clear.

Retention is not automatic and must be agreed in the contract. Consider whether an agreed retention or meaningful final balance should remain until practical completion, handover documents and the contractual snagging or defects arrangements have been addressed. Residential-owner contracts do not all follow the same statutory payment regime, so the written contract is important.

10. Use a Written Contract That Matches the Project

A substantial residential project should not rely on messages and assumptions. A written contract should identify the parties and incorporate the agreed drawings, quotation, scope, specifications and other contract documents.

It should also address:

the contract price and whether VAT is included;

• start, programme and completion arrangements;

• payment, valuations, deposits, retention and final payment;

• access, occupation, insurance and responsibility for approvals;

• design responsibilities and information to be provided by others;

• variations, delay, extension of time, suspension and termination;

• practical completion, certificates, snagging and defects;

• how disputes will be handled.

JCT publishes Home Owner contracts for domestic building work, including versions for projects with or without a consultant administering the contract. The appropriate form depends on the size, design responsibilities and management arrangements, and professional advice may be worthwhile for a major project.

The Consumer Rights Act 2015 requires a trader to perform services with reasonable care and skill. Information given about the service can also become binding. A clear written agreement remains the best way to record what that service is.

Where a contract is concluded at the homeowner's property, online or by telephone, a 14-day cancellation period may apply under the Consumer Contracts Regulations, subject to the circumstances and exceptions. If the homeowner expressly asks for work to begin during that period, the paperwork should explain the consequences of cancellation after work has started.

11. Agree How Variations and Hidden Conditions Will Be Managed

Changes are common. A homeowner may select a different product, the design team may issue revised information, or opening up may reveal a condition that could not reasonably have been confirmed earlier. The contractor should not treat every cost outside its original scope as an informal extra.

• The agreed process should confirm:

• who has authority to instruct a change;

• how the price or method of valuation will be established;

• whether the programme is affected;

• what must be approved in writing before proceeding;

• how urgent safety or protection work will be handled.

Where important conditions remain concealed and the client's timing allows, Alt Construction may recommend a separately scoped first stage for strip-out, agreed non-structural demolition and targeted opening-up. Necessary asbestos checks, service isolations and structural safeguards must be addressed before that work.

The architect and structural engineer can then review the exposed building and update the technical design and Scope of Works before the main construction is priced. If the stages are contractually separate, the client remains free to ask Alt Construction and other contractors to tender for Stage 2. This approach is not suitable for every programme, but it can reduce uncertainty where the existing conditions justify the additional time.

12. Judge the Communication Before Construction Starts

The quotation stage is a useful preview of the working relationship. A residential project can last for months and involve many decisions about cost, design, products, access and unforeseen conditions. Communication is part of project delivery, not an optional extra.

• Consider whether the contractor:

• asks relevant questions and reviews the available information;

• explains uncertainty rather than hiding it;

• gives clear answers and follows important points in writing;

• distinguishes a budget estimate from a formal quotation;

• avoids promising a fixed outcome where major information is missing;

• is realistic about availability, programme and decision deadlines.

Fast replies are helpful, but the quality and consistency of the information matter more than speed alone. The homeowner should understand who to contact, how often updates will be provided and how urgent decisions will be escalated.

Red Flags Worth Investigating

No single issue automatically proves that a builder should be rejected. Several warning signs together should, however, prompt further questions before any contract or payment.

• pressure to sign or transfer money immediately;

• quotation, contract, insurance and bank details that identify different businesses without explanation;

• a substantially lower price with no clear scope-based reason;

• large upfront payments or a heavily front-loaded valuation schedule;

• very low allowances for second fix, finishes, decoration, testing or completion;

• a vague scope combined with an unconditional fixed-price promise;

• reluctance to provide insurance, project evidence or references;

• refusal to document payment terms, changes or exclusions;

• technical and safety questions repeatedly being dismissed

Why Some Homeowners Come Back to Alt Construction After Choosing a Lower Quote

Alt Construction has been contacted by homeowners who originally appointed another contractor because the overall quotation was lower. When problems later developed, they returned to us because they remembered that our earlier advice had been open, detailed and available.

When we review the original quotation and payment history, the problem is sometimes visible from the beginning: important work was omitted, allowances were too low, or too much value was collected during the early stages while too little remained to fund completion.

This does not mean every lower quotation is wrong, or that the highest price is automatically best. It means the homeowner should compare the credibility of the scope, price distribution, payment profile and delivery plan - not simply the figure at the bottom.

The Alt Construction Approach

Before asking a client to rely on a construction quotation, our preferred process is to:

• review the address, brief, drawings, planning position and technical information;

• identify material gaps that should be resolved by the appointed designer, engineer or specialist;

• assess access, occupation, parking, storage, deliveries, waste and site management;

• define inclusions, exclusions, assumptions, provisional allowances, client-supplied items and VAT;

• use a payment structure that reflects genuine progress and leaves a credible value for completion;

• record variations and programme effects in writing;

use a separate investigation stage only where the uncertainty and client's timing justify it.

A detailed quotation cannot remove every construction risk. It should, however, make the known scope and remaining uncertainty visible enough for the client to make an informed decision.

A Simple Builder Comparison Checklist

• The contracting business and VAT details are verified.

• The builder has relevant experience and a credible delivery team.

• Recent projects, references and reviews have been checked.

• The available design information is sufficient for the quotation stage.

• All quotations have been compared against the same scope.

• Inclusions, exclusions, assumptions, allowances and VAT are clear.

• The quotation is not unfairly front-loaded.

• The remaining balance after each payment can reasonably fund completion.

• Insurance, safety and specialist responsibilities are understood.

• A suitable written contract and variation process are ready.

• The programme, site manager and reporting arrangements are credible.

• The contractor's communication gives confidence before work starts.

Frequently Asked Questions

How many builder quotations should I obtain?

There is no number that guarantees a good decision. Three well-prepared quotations based on the same information are normally more useful than six prices based on different assumptions.

Should I choose the cheapest builder?

Not automatically. Compare the scope, exclusions, allowances, experience, technical understanding, payment profile and delivery plan. A lower price can be completely genuine, but the reason for the difference should be understandable.

How detailed should a builder's quotation be?

Detailed enough to understand what is included, excluded, assumed and provisional, who supplies key items, whether VAT is included and how the contract value is divided. It does not need to list every screw.

How can I tell whether a quotation is front-loaded?

Compare the value placed in the early stages with the realistic work still required later. If most of the price is collected through preliminaries, demolition and structure while only small amounts remain for second fix, finishes and completion, ask for the allocation to be explained.

Is it normal to pay a deposit?

A reasonable deposit can cover booking, mobilisation or agreed procurement. Its purpose, amount and treatment should be written down. Paying the whole project value before work starts creates unnecessary risk.

Should I pay for materials before they arrive on site?

Sometimes advance payment is commercially necessary for bespoke or long-lead products. The contract should address ownership, identification, storage, insurance and what happens if the order or project does not proceed.

Do I need a written contract for an extension?

A written contract is strongly advisable. It records the agreed documents, price, payment process, time, responsibilities, changes, completion and dispute arrangements.

Are online reviews enough?

No. Use them alongside company and VAT checks, relevant project evidence, references, insurance documentation and a direct assessment of the quotation and proposed team.

Planning a London Extension or Renovation?

If you would like Alt Construction to assess whether your project is ready for a meaningful construction quotation, send us:

• the property address;

• available drawings and structural information;

• a brief description of the intended work;

• your working budget range;

• your preferred timing.

Email info@altconstruction.co.uk or visit www.altconstruction.co.uk. We can then advise whether a site visit, further technical clarification or another next step would be most useful.

Official Sources and Further Reading

This guide reflects official information available in August 2026 and Alt Construction's practical experience. Consumer, company and construction requirements can change and should be confirmed for the individual project.

Companies House - Find and Update Company Information - free company status, officer, filing and document information.

Companies House - Verifying Your Identity - current identity-verification requirements and implementation.

HMRC - Check a UK VAT Number - verification of a VAT number and its registered business name and address.

Consumer Rights Act 2015 - the statutory requirement for services to be performed with reasonable care and skill.

Consumer Contracts Regulations 2013 - information and cancellation requirements for relevant distance and off-premises contracts.

HSE - Domestic Clients: Roles and Responsibilities - how CDM duties operate for domestic construction clients.

HSE - Principal Contractors: Roles and Responsibilities - construction-phase planning, management and coordination duties.

JCT - Home Owner Contracts - standard-form domestic building contracts with and without a consultant.

Citizens Advice - Before You Get Building Work Done - consumer checks covering quotations, payments, deposits and documentation.

TrustMark - How Do I Find the Right Tradesperson? - homeowner guidance from the Government Endorsed Quality Scheme.

Important: This article provides general information, not project-specific legal, financial, insurance, architectural or construction advice. Appropriate professional advice should be obtained where required.