Hidden Costs of a House Extension in London: 15 Expenses Homeowners Often Forget

Building a house extension can create valuable living space and may increase the value and enjoyment of your property. However, the total project budget is rarely limited to the builder's main construction quotation.

Homeowners may also need to allow for professional fees, statutory approvals, VAT, surveys, London access costs, client-supplied finishes and unforeseen existing conditions. These costs are not necessarily being "hidden" by a contractor. Many sit outside the agreed construction scope or cannot be confirmed until the design and condition of the property are better understood.

This guide explains 15 costs that London homeowners commonly overlook, when they may apply and how sensible pre-construction planning can reduce uncertainty.

Quick answer: what should you budget for beyond the building work?

Before committing to a house extension, consider the complete project cost rather than comparing only the headline figures from builders.

Budget item	Practical guide
VAT	Most extension work is normally subject to VAT at 20%. A reduced rate may apply in limited qualifying circumstances.
Planning application	Where householder planning permission is required, the application fee in England is £548 as at July 2026, plus any applicable submission service charge.
Thames Water	Current Build Over application fees start from £130. CCTV surveys, repairs, legal work or sewer diversion can cost considerably more.
Contingency	Around 10-15% of the construction budget is a sensible starting point, adjusted for the condition and complexity of the property.
Professional and third-party costs	Architects, engineers, surveyors, Building Control and other specialists should normally be budgeted separately unless expressly included.

The important question is not simply, "What is the builder charging?" It is, "What is the likely total investment required to complete the project properly?"

Why house-extension budgets increase

A builder's quotation normally covers the scope of construction work described in that quotation. It may not include professional appointments, authority fees, client purchases or work that cannot yet be properly defined.

Work requested or required beyond the agreed construction scope may become a variation. Other costs may be paid directly by the homeowner to consultants, authorities, surveyors or suppliers.

In our experience, budgets usually increase for four main reasons:

• the design or specification was incomplete when prices were requested;

• the existing condition of the property was not sufficiently understood;

• important client selections or decisions were made late;

• third-party, statutory and logistical costs were left outside the initial budget.

No amount of planning can remove every unknown, particularly in an older London property. The objective is to identify the risks that can reasonably be investigated and to deal transparently with those that remain.

Hidden costs before construction begins

1. VAT

VAT is one of the largest items homeowners overlook when comparing extension quotations.

Most house-extension work to an existing occupied dwelling is normally subject to VAT at the standard rate of 20%. Every quotation or budget should therefore state clearly whether its figures are inclusive or exclusive of VAT.

A reduced VAT rate of 5% may apply to qualifying renovation or alteration work where qualifying residential premises have not been lived in for at least two years immediately before the work begins. HMRC confirms that qualifying work can include constructing an extension as part of the renovation of an eligible empty property.

The reduced rate is not an optional discount that a client can simply request. Evidence of the vacancy period may be required and some services, including professional fees, landscaping and the erection or dismantling of scaffolding, remain standard-rated. Eligibility and any necessary division between different VAT rates should be confirmed before the quotation and contract are issued.

2. Planning applications and professional advice

Some extensions require householder planning permission. Others may fall within Permitted Development, sometimes with prior approval, while a Lawful Development Certificate may still be sensible to formally confirm the lawful position.

The correct route depends on the property, the proposed size and design, previous alterations, local planning restrictions and whether the building is listed or in a conservation area.

Where a householder planning application is required, the application fee in England is £548 as at July 2026. Planning Portal service charges may also apply. The fees charged by an architect, planning consultant or other professional for preparing and managing an application are separate.

3. Technical drawings and specifications

Planning drawings communicate the proposed appearance and general design. They are not necessarily a complete set of instructions for pricing and construction.

At ALT Construction, we regularly receive planning drawings that do not include enough information to confidently coordinate and price the work. Common gaps include:

• construction build-ups and key junction details;

• confirmed dimensions and setting-out information;

• drainage and service routes;

• structural information;

• door, window and rooflight specifications;

• responsibility for finishes and specialist items.

Although a Building Notice route may be available for some domestic work, the contractor still needs sufficient technical information to plan, price and build the project properly. A coordinated technical package normally gives the client a more reliable basis for comparing quotations.

4. Structural engineering and temporary works

Structural calculations and details are commonly required where an extension involves new foundations, steel beams, columns, significant openings or alterations to load-bearing construction.

The structural engineer may need to specify:

• beam and column sizes;

• bearings, padstones and connections;

• foundation requirements;

• lateral restraint and stability details;

• the sequence or design of temporary support where required.

Temporary propping, fabrication drawings or specialist temporary-works advice can also carry additional cost. The builder can comment on practical buildability, but formal structural design must come from the appointed engineer.

5. Building Control fees and completion requirements

Planning permission and Building Regulations approval are separate matters. Most conventional house extensions require Building Regulations approval even where planning permission is not needed.

The client will normally need to budget for:

• the Building Control application;

• plan checking and site inspections;

• any specialist testing or supporting information;

• completion documentation and the final certificate.

Charges vary between local-authority Building Control services and registered building-control approvers. Before work starts, the project team should also confirm the relevant client, designer and contractor dutyholder roles and the information needed at completion.

6. Party Wall notices and surveyors

The Party Wall etc. Act 1996 may apply where the proposed work:

• affects an existing party wall or party structure;

• involves a new wall at or astride a boundary;

• includes qualifying excavations within three or six metres of a neighbouring structure.

A Party Wall surveyor is not automatically required. If the adjoining owner receives a valid notice and consents, the work may proceed without a Party Wall Award. If there is a dispute or no response, the owners may use one agreed surveyor or appoint separate surveyors.

The process should be considered early because notices, surveys and awards can affect both the budget and the intended starting date.

7. Drainage surveys and Thames Water approval

Public sewers and lateral drains frequently run beneath or close to London gardens.

Thames Water advises that an application is likely where work is proposed within three metres of a public sewer or within one metre of a public lateral drain. Depending on the pipe and proposal, the project may require a Build Over Agreement, CCTV drainage surveys, protective details, repairs or diversion.

As at July 2026, Thames Water application fees start from £130 for Class 1, £490 for Class 2 and £585 for Class 3, with additional legal costs starting from £1,000 for Class 3 cases. These fees are updated periodically, and the cost of surveys or physical drainage work is separate.

8. Targeted surveys and opening-up investigations

A formal ground investigation is not required for every house extension. However, targeted investigations can be valuable where there are genuine warning signs or insufficient information.

Depending on the property and proposed work, these may include:

• trial pits to establish the depth and condition of existing foundations;

• CCTV drainage surveys;

• utility searches;

• arboricultural or root-related advice;

• controlled opening-up of previous alterations;

• an asbestos refurbishment survey for affected areas of an older property.

For buildings constructed or refurbished before 2000, asbestos-containing materials should be considered before work disturbs the existing fabric.

The purpose of these investigations is not to create unnecessary cost. It is to resolve identifiable risks before they become expensive problems during construction.

9. Property consents, legal matters and insurance

Planning permission does not override private property rights or every other consent.

Depending on the property, the homeowner may need to consider:

• freeholder or management-company consent;

• a licence to alter;

• restrictive covenants;

• lender notification;

• access agreements with neighbours;

• notification to the home insurer;

• suitable cover while major building work is taking place.

These issues are particularly relevant to leasehold properties, converted buildings and projects where access is required through neighbouring land.

Hidden costs during construction

10. London access, scaffolding and site logistics

London logistics can materially affect the cost of an extension. Restricted access, controlled parking, narrow roads and limited storage often increase the labour and planning required for deliveries and waste removal.

Allowances may be needed for:

• scaffolding and temporary protection;

• highway or pavement licences;

• skips, grab collections and waste management;

• parking suspensions, permits or daily parking;

• smaller or repeated deliveries;

• crane, HIAB or specialist lifting arrangements;

• carrying materials through an occupied property;

• off-site storage where there is no secure space on site.

These matters should be assessed during the site visit and quotation stage rather than left until construction begins.

11. Service capacity, diversions and upgrades

An extension may require existing services to be altered, but an upgrade should not be assumed automatically. The need depends on the capacity and condition of the existing systems and the proposed design.

Possible costs include:

• relocating water, gas, electrical or drainage services;

• assessing boiler and heating capacity;

• upgrading electrical distribution where justified;

• relocating meters or incoming supplies;

• providing new drainage connections;

• managing utility-company applications and lead times.

Underfloor heating, additional radiators and premium electrical features should be identified as design choices rather than presented as unavoidable hidden costs.

12. Existing conditions revealed during the work

Opening walls, lifting floors and excavating foundations can reveal conditions that were not visible during a normal survey or site visit.

Examples include:

• decayed timber;

• unsuitable or undocumented previous alterations;

• inadequate lintels or unsupported masonry;

• damaged brickwork;

• concealed services;

• drainage defects;

• foundations that are unsuitable for the proposed work.

Targeted surveys and opening-up can reduce this risk, but they cannot always remove it completely. Where additional work becomes necessary, the reason, cost and programme effect should be explained and agreed in writing.

13. Client changes, finish selections and procurement

Late changes are one of the most common causes of additional cost and delay.

Typical examples include:

• larger doors or additional rooflights;

• revised layouts or lighting positions;

• different flooring or tiling;

• upgraded kitchens or sanitaryware;

• bespoke joinery;

• changes made after materials have been ordered or work has been completed.

Client-supplied kitchens, tiles, sanitaryware, flooring, light fittings, appliances and non-standard finishes should be clearly identified in the quotation. The parties should also agree who is responsible for ordering, checking quantities, delivery, storage and replacement of missing or damaged items.

ALT Construction can assist with sourcing and procurement where agreed, but the responsibility and any associated handling cost should be stated in advance.

Costs towards completion

14. Temporary living arrangements and reinstatement

Many homeowners remain in their property during an extension, but this is not always practical throughout the entire programme.

The overall budget may need to include:

• temporary accommodation;

• furniture removal or storage;

• a temporary kitchen or alternative meal arrangements;

• additional protection and phased working where the house remains occupied;

• reinstatement of patios, lawns, fences and pathways;

• decorating or flooring in adjoining rooms so the old and new areas connect properly.

The builder's quotation should state which reinstatement and existing-room works are included. Remaining in occupation may reduce accommodation costs, but it can require additional phasing, protection and coordination.

15. A realistic contingency

Even a well-designed and carefully investigated project can encounter unforeseen conditions.

As a general starting point, homeowners should consider a contingency of around 10-15% of the construction budget. A newer, well-documented and fully designed property may justify a lower allowance, while an older or poorly documented building may require more.

The contingency should be kept separate from:

• VAT;

• known professional and statutory fees;

• client-requested upgrades;

• provisional allowances already identified in the quotation.

If contingency money is spent on additional construction work, VAT may also be payable on that expenditure.

When a two-stage approach can reduce uncertainty

Where important conditions remain genuinely unclear and the client can allow time for investigation before committing to the main build, ALT Construction may recommend dividing the project into two clearly defined stages.

This approach is not required for every project. It is most useful where a responsible quotation for the main work cannot be prepared without first exposing the relevant parts of the existing building.

Stage 1: agreed strip-out, demolition and investigation

The first stage covers an agreed programme of enabling works. Depending on the project, this may include the extent of strip-out, opening-up and demolition needed to expose the existing structure and services.

Before Stage 1 begins, the necessary asbestos checks, service isolations, structural advice, temporary support and statutory requirements must be addressed.

Once the relevant areas are exposed, the architect and structural engineer, where required, can inspect the actual conditions and incorporate the findings into a more accurate technical design and Scope of Works for Stage 2.

Stage 2: defined main construction works

The main work can then be priced against clearer information. This can reduce unforeseen variations, improve programming and allow the client to make decisions before committing to the main construction cost.

The two stages may also be contractually separate. A client may appoint ALT Construction for Stage 1 without being obliged to appoint us for Stage 2. Once the design and Scope of Works are clearer, the client is free to ask ALT and other contractors to quote on the same information.

We believe this is a fair and transparent approach. It avoids locking a client into a main construction contract while important parts of the scope remain uncertain, and it gives everyone a more consistent basis for pricing and planning.

The ALT Construction approach

ALT Construction works as the building contractor while providing practical advice on buildability, logistics, sequencing and cost risk. Formal architectural and structural design remains with the client's appointed professional team.

Our typical process is:

1. The client sends the project address, brief, available drawings, expected budget and preferred timing.

2. We review the available information and identify obvious gaps that could affect pricing or construction.

3. We visit the property where appropriate to assess access, logistics, occupation, protection and the visible condition of the site.

4. The appointed architect, engineer and other specialists complete the technical information and approvals required for the project.

5. If significant concealed conditions remain and the programme allows, we may propose a separately scoped Stage 1 investigation before the main quotation is finalised.

6. We prepare a quotation with the scope, inclusions, exclusions, assumptions, provisional allowances, client-supplied items and VAT clearly stated.

7. Before starting, the parties confirm the contract, programme, approvals, procurement responsibilities and important client decisions.

8. During construction, the site manager coordinates the work, Building Control inspections and agreed programme. Variations are recorded and agreed in writing.

9. At completion, testing, certificates, snagging, handover and the Building Control completion process are addressed as applicable.

Our objective is not to pretend that every risk can be eliminated. It is to make the known scope clear, investigate material uncertainties where practical and communicate openly when existing conditions or client decisions change the work.

Frequently asked questions

What is the biggest hidden cost of a house extension?

There is no single answer for every property. VAT is often the largest amount omitted from an early budget. During construction, the most significant additional costs commonly arise from incomplete design information, existing conditions, drainage problems or client changes.

Is VAT always charged at 20%?

Most extension work to an existing occupied dwelling is normally standard-rated at 20%. A 5% rate may apply to qualifying renovation or alteration work to eligible residential premises that have not been lived in for at least two years. The conditions, evidence and treatment of individual services must be checked before invoicing.

Are planning drawings enough for a construction quotation?

They may be enough for an early budget indication, but they rarely provide all the information needed for a detailed and dependable construction quotation. Structural design, specifications, drainage, services and key construction details may still be required.

Is a Party Wall surveyor always required?

No. The Act may require notice to be served, but a surveyor is not automatically needed if the adjoining owner gives written consent. Surveyors normally become necessary where a dispute arises or the adjoining owner does not respond.

How much contingency should I keep?

Around 10-15% of the construction budget is a sensible general starting point. The appropriate allowance depends on the age and condition of the property, the completeness of the design and how much investigation has been possible.

When is a two-stage construction approach useful?

It can be helpful where important conditions are concealed, the main scope cannot yet be responsibly defined and the client has enough time to complete an investigative stage before committing to the main works.

What should I send ALT Construction for an initial review?

Send us the property address, a short project brief, all available drawings and specifications, your preferred timing and a realistic indication of the available budget. This allows us to understand the project and identify the most useful next step.

Final thoughts

A successful extension is not simply about selecting the lowest initial quotation. It is about understanding what has been included, what remains outside the construction scope and which risks should be investigated before work begins.

Clear technical information, realistic allowances and early decisions give the client and contractor a better chance of controlling cost and programme.

If you are planning a house extension in London, send ALT Construction your drawings and project brief. We can review the available information, discuss the practical construction considerations and help you understand the next steps towards a reliable quotation.

Official guidance

HMRC: Buildings and construction - VAT Notice 708; GOV.UK: Building Regulations approval; GOV.UK: Party Wall etc. Act explanatory booklet; Thames Water: Build Over Agreements; HSE: Identifying asbestos before refurbishment; Planning Portal: Application fees.