How to Choose a House Extension Builder in London

Photograph: a completed BCS house extension project in Greenwich, London.

Most extension disputes are decided before anyone lifts a tool. They are decided by what was written down, what was checked and what was assumed. A London homeowner appointing an extension builder is usually comparing three documents that describe three slightly different jobs, from three businesses whose registration, insurance and site-safety position they have not verified.

This guide is the sequence we would want applied to us. Work through it with every builder quoting for your project, including BCS. If a builder objects to any of it, that is useful information in itself.

Settle the scope before you look at prices

You cannot compare builders until the scope is settled, because a builder can only price what has been decided. If the layout, the structural approach, the glazing, the kitchen or the floor finish are still moving, each quote is pricing a different house.

The approvals position matters just as much. A project that falls within permitted development, one that needs householder planning permission and one that needs prior approval are three different programmes. Read our guide to planning permission for London extensions before you ask anyone to price the work, and use the extension timeline guide to see which decisions have to land before a start date means anything.

TrustMark puts the same step first: have a clear idea of the work you want done, and research the probable cost and timescales, before you approach anyone. Good people are in demand, so you may have to wait for the right one.

Check the business is what it says it is

Citizens Advice is blunt about this. Check what a trader or their website tells you, and make sure they are not misleading you. In practice that means:

• ask for a business card or letterhead, or full contact details, and ring the business number to confirm the business exists and that the person works for it;
• ask to see proof of qualifications — for example an NVQ in construction, or a Construction Skills Certification Scheme card;
• if a builder claims membership of an approved trader scheme, a competent person scheme or a trade association, check it on that scheme’s own register rather than on the builder’s website. Apply that to us as readily as to anyone else;
• be wary of a contact who gives only a mobile number. They can be hard to reach when a problem comes up.

Take references you can actually contact

Ask for two or three recent examples of similar work, and for the contact details of the people it was done for. Citizens Advice is clear that written references are not always genuine, so it is worth making the call yourself.

TrustMark suggests what to ask those previous customers: what was the tradesperson like to deal with, were they tidy, did they turn up when they said they would, and did they keep to the price quoted and the time they said the job would take.

A builder who will not give references is telling you something. Citizens Advice describes it as a sign they might be dishonest, and advises avoiding them.

Get quotes, not estimates

A quote is a fixed price, so you know what you are getting and what it will cost. An estimate is a rough guess and you may end up paying more. Do not rely on a verbal quote, and be cautious about a builder who will not put one in writing.

Citizens Advice sets out what a quote should include:

• a fixed total price, not a daily rate;
• a breakdown of all the work to be done and the materials needed;
• separate costs for each material and each part of the work;
• how long the price is valid for;
• whether the price includes VAT;
• when the price can go up — for example, only if you agree to extra work.

TrustMark advises obtaining three quotes before starting any work, so you have a range to compare. If one price sits far below the others, treat it as a question rather than a bargain: Citizens Advice notes it can mean the builder does not have the right skills or experience, is not being honest, or simply is not quoting for the same work. Our renovation quote comparison checklist gives you a way to line three documents up against each other, and the extension cost guide explains what moves a London budget.

If you are offered a daily rate instead of a fixed total, get three things in writing: how many days the work will take, how many hours count as a day, and the point at which the builder needs your agreement to work more days.

Insurance — yours as well as theirs

Start with your own buildings insurer and tell them what you are planning. Then check the builder. Citizens Advice names four covers that may be relevant to building work:

• public liability insurance;
• employers’ liability insurance;
• professional indemnity insurance;
• contractors’ all-risk cover.

Not all four apply to every project. Ask which apply to yours, ask to see them, and ask what they would and would not cover if something went wrong on your site.

Get it in writing before anyone starts

Always use a written contract. Both sides then have a clear written agreement about what is going to be done, when, and how much it will cost, and it offers you protection if something goes wrong. Agree any change to it in writing as well. Check that the contract covers the essential details, how you will pay, and any guarantees or warranties being offered.

On payment, TrustMark is direct: do not simply go with the cheapest option, and do not pay in cash rather than take a properly invoiced job which may include VAT. Only pay for work that has been done, rather than by advance payments — though you may reasonably be asked for a contribution towards materials when they are bought.

Who holds the site-safety duties on your project

This is the question almost no builder’s website answers, and it is worth asking in writing.

Under the Construction (Design and Management) Regulations 2015, a domestic client — anyone having construction work carried out on their own home, or a family member’s home, that is not part of a business — normally has their client duties passed to someone else. HSE sets out where they go:

• on a single-contractor project, to the contractor, who takes on the client’s legal duties in addition to their own;
• on a project with more than one contractor, to the principal contractor. If you have not appointed a principal contractor, those client duties must be carried out by the contractor in control of the construction work.

There is one exception worth knowing. If you appoint an architect or other designer on a project involving more than one contractor, you can ask them to manage it and take on the client duties instead of the principal contractor. They then take on the responsibilities of principal designer — but only if they have a written agreement with you confirming it. Without that written agreement, the client duties pass automatically back to the principal contractor.

Building control is not planning permission

Building regulations approval is different from planning permission, and you might need both. GOV.UK is explicit about that, and it is a common and expensive misunderstanding.

Approval comes from a building control body. You can choose either your local authority’s building control department or a private registered building control approver. Ask which route the builder proposes, who is making the application, and who pays the fees.

There is one shortcut, but it is narrower than people expect: if you use someone registered with a competent person scheme, you do not need to get approval yourself for the types of work that scheme covers. Building an extension is not one of them, so extension work still needs a building control body.

The consequences of skipping it are real. The person doing the work could be prosecuted and fined for not complying with building regulations. The relevant building control body could make you pay to fix faulty work. And without approval you will not have the certificates of compliance you may need when you come to sell.

GOV.UK also notes that from 1 October 2026, applications for building control approval and initial notices for certain residential buildings may be subject to a tax called the Building Safety Levy. Whether it applies to your project is a question for your building control body rather than something to assume either way.

Warning signs

• no written quote, or a refusal to put the scope in writing;
• no references, or references you cannot contact;
• a mobile number and nothing else;
• pressure to pay in advance, or to pay cash rather than take an invoice;
• a price far below every other quote you have received;
• a claimed scheme membership you cannot find on the scheme’s own register;
• vagueness about who holds the CDM 2015 client duties, or about which building control route is being used.

The questions that matter more in London

Every project raises these. A constrained London street makes them decisive, and they are worth asking before you choose between quotes.

• Access and deliveries. How will materials arrive and waste leave, and where will a skip or vehicle stand?
• Storage and protection. Where will materials be kept, and how will the rest of the house be protected while work is going on?
• Neighbours. You must tell your neighbours if you want to carry out any building work near or on a shared property boundary. Party wall matters are separate from both planning permission and building regulations approval, so ask early what the builder expects to be needed and take advice from a party wall surveyor if a neighbour does not agree.
• Which borough. Local rules vary. Confirm the position for your address with the council rather than assuming a national rule settles it.
• Floor area. The Planning Portal notes that work adding over 100 square metres of floor space may be liable for a charge under the Community Infrastructure Levy. Check that with the borough before you budget.

How BCS works

We manage extensions from design through to build. We prepare and coordinate the information a planning or permitted-development route needs, and we keep the process moving where it is within our control. The decision itself belongs to the local planning authority.

We agree a project-specific programme once the scope, information and site conditions are clear, rather than offering a generic completion date before the property has been surveyed. After reviewing the drawings, site conditions and finish specification we prepare a detailed quote, and we identify allowances or provisional sums where an item cannot yet be fixed.

Insurance and workmanship warranty details are available on request and should be checked against the scope of your individual project before work starts. Apply the checks in this guide to us in exactly the way you would apply them to anyone else quoting.