House Extension Builder in London: How to Choose One
The best house extension builder for most London homeowners is a single team that designs, gets the approvals for, builds and finishes the work under one contract. You deal with one point of responsibility instead of an architect, an engineer, a builder and a string of trades who each blame the last. Kovacs Construction is a family-run design and build firm with 42 years of experience in London, and this guide explains how to judge any builder: what to ask, what the money looks like and where extensions go wrong.
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Plan Your London Extension With One Team
Kovacs Construction brings design and building work together for London extensions, helping homeowners avoid coordinating multiple contractors.
Key Takeaways
A design and build contractor takes the project from concept to handover under one contract, which means one party answers for the result.
Budget by the square metre. The HOA’s 2026 guide puts London single-storey extensions at £2,300 to £3,400 per m².
Get at least three written, itemised quotes based on the same drawings, and never pay everything up front.
Planning permission, building regulations and party wall notices are three separate processes. Your builder should know which apply to you.
Allow nine to fifteen months from first idea to completion on a straightforward extension, with the build itself taking roughly four to eight months.
Table of Contents
What a house extension builder actually does
A builder who only builds expects finished drawings from you. A design and build contractor starts earlier. According to Homebuilding magazine, a design and build package brings design and construction under a single contract, with one organization responsible for delivery from concept to completion. That organization manages the appointments and coordinates the design, including structural design, insulation, drainage layouts and the construction details that prove compliance with building regulations.
Once full building regulations approval is in place, the same contractor coordinates the trades, runs the programme, oversees quality and health and safety, and hands the finished work over to you.
For a homeowner this changes three things:
One contract. The price, programme and scope sit in one document instead of several.
One answer to “whose fault is this?” When a structural detail clashes with a kitchen layout, the people who drew it and the people who build it are the same team.
Less coordination for you. You are not chasing an engineer for a calculation while a bricklayer waits on site.
The alternative route is to hire an architect first. RIBA explains that a chartered architect can take you from the brief through planning approval and construction details, then help you find a builder and check the work on site. That suits some projects, but it means two or more appointments, two sets of fees and a handover between designer and builder. A turnkey firm removes that handover.
What a London extension costs per square metre
Builders quote by the project, but you can sanity-check any figure by the square metre. Published guides differ by region and by year, so treat them as a range, not a promise.
Source | Type | Cost per m² |
|---|---|---|
Bury Council homeowner guide (2022) | Single-storey | £1,000 to £2,100 |
Bury Council homeowner guide (2022) | Two-storey | £1,300 to £2,500 |
HOA guide (2026), UK | Single-storey | £1,800 to £3,000 |
HOA guide (2026), London | Single-storey | £2,300 to £3,400 |
The HOA also gives £48,000 plus VAT as the average for a standard 20 m² single-storey extension across the UK. London sits above that average, so a 20 m² rear extension in the capital will usually come in higher.
Two points help when you read a quote:
Two storeys cost less per m². The HOA notes that a two-storey extension runs at around £1,900 per m² for a standard finish, because the roof and foundations do not double when you add a floor. If you need more space, building up can be better value than building out.
Finish matters more than footprint. Glazing, kitchen specification, underfloor heating and structural steel all move the figure. This is why a per-m² number is a starting point only and why an itemised quote matters.
If you are weighing a loft instead of a rear or side extension, Kovacs Construction publishes its London ranges on its FAQ page: £40,000 to £55,000 for a Velux or rooflight conversion, £50,000 to £75,000 for a dormer, and £65,000 to £95,000 for a mansard or hip-to-gable conversion.
How to compare quotes properly
The cheapest quote is rarely the cheapest job. Bury Council’s guidance tells homeowners to get at least three quotes, agree a written contract and a payment schedule, and never pay everything in advance. It also warns that a builder who can start immediately or submits an extremely low bid may cut corners or add charges later.
Homebuilding’s guide to finding a builder lists what a good estimate contains. Check each quote for:
a detailed breakdown that separates materials from labor;
a health and safety report;
an allowance for waste and skips;
a clear statement of whether VAT is included;
a project management fee, shown rather than buried.
The Hepburn Architects guide makes a point that saves a lot of arguments: every contractor should price the same coordinated set of drawings and information. If one builder prices from a sketch and another from full structural drawings, you are comparing assumptions, not prices.
Ask each builder for a schedule of works that says exactly what is included. Kitchens, flooring, decoration, electrics and making good to the existing house are the usual gaps. A quote that stops at “shell complete” leaves you to find, price and coordinate the rest yourself.
Questions to put to any builder
Who will be on site every day, and are they employed directly or self-employed subcontractors? Homebuilding recommends asking this because it tells you what you are paying for.
What other jobs are they running at the same time?
Can they show you completed work and put you in touch with former clients?
Who handles structural engineering, building control and party wall paperwork?
Bury Council’s guide adds that a good builder is usually busy with work lined up months ahead, and that a recommendation from someone you know is the best reassurance. A diary that is full is a better sign than an offer to start next week.
Planning permission, building regulations and party walls
These are three separate things, and homeowners often assume one covers the others.
Planning permission. RICS advises that in England and Wales the first step is the Planning Portal, the government’s planning information site. Many modest extensions fall under permitted development, meaning national rules allow them without a full application, but the limits are tight and London has many exceptions. If you live in a listed building or a conservation area, RIBA states that you need consent from the local authority on top of building regulations approval.
Building regulations. These apply to almost every extension regardless of planning. RICS explains that they cover structural safety, fire protection, energy efficiency and ventilation. When the work finishes, get a completion certificate from the building control body. Without one you can face problems when you sell.
Party wall notice. If you share a wall with a neighbour or dig close to their property, you need to serve formal notice. RIBA says this must be done at least two months before work starts if you share a wall or excavate within three or six metres of a neighbouring building. Terraced and semi-detached London houses very often trigger it.
A step the law gives to an authority is part of the process, not something a builder can skip. A good design and build firm plans the programme around those approvals so that nobody waits on site for paperwork.
The stages and how long they take
Extensions take longer than most owners expect, mostly before a brick is laid. The Hepburn guide gives these typical durations:
Stage | Typical duration |
|---|---|
Initial consultation and brief | 1 to 2 weeks |
Measured survey | 1 to 3 weeks |
Concept design | 3 to 6 weeks |
Planning submission preparation | 2 to 4 weeks |
Planning determination | Around 8 weeks from validation |
Building regulations and structural design | 4 to 8 weeks |
Contractor pricing and appointment | 4 to 8 weeks |
Construction | About 4 to 8 months |
Completion and final certification | 1 to 3 weeks |
Its estimate for a straightforward extension is nine to fifteen months overall. The local authority normally has eight weeks to decide a valid householder application, and that can be extended by agreement.
The row that shrinks most under one contract is “contractor pricing and appointment”. When the firm that designs the extension also builds it, you are not tendering finished drawings to three builders and waiting for them to come back. You still compare quotes at the start, but the design, structural work and build price come from one team.
Limiting disruption and surprise costs
Most overruns come from the same few places:
Unclear scope. Fix it with the schedule of works described above.
Changes after the build starts. Settle the kitchen layout, electrical points and finishes before the contract is signed. Late changes are the most common source of extra charges.
Hidden ground or structural issues. A proper survey and structural design before pricing reduces what can surprise anyone later.
Payments that run ahead of the work. Agree a schedule tied to stages, in writing.
On living through the build, RIBA suggests telling neighbours well ahead, since the work is noisy and a skip on the street is an inconvenience. It also tells homeowners to check their house insurance, because construction that changes the structure or value of your home may affect buildings and contents cover. Call your insurer before work begins.
Why one team beats several contractors
The usual way to build an extension is to assemble it: a designer, a structural engineer, a builder, then electricians, plumbers, plasterers and a kitchen fitter. Each is competent, and each is responsible only for their own part. The gaps between them are where delays and disputes live.
Kovacs Construction works the other way. We are a family-run team with 42 years of experience, and every project is managed by that single team from design to completion, with no coordinating of multiple subcontractors left to you. Our approach to pricing is transparent, with no hidden fees, so the number you agree is the number you plan around. That is the kind of arrangement the Homebuilding and Bury guidance points toward: one contract, a clear scope and one party accountable for the finished work.
Working with Kovacs Construction
We design and build residential projects across London: kitchen extensions, loft conversions, basements, high-end renovations, remodelling and additions, and structural repairs and reinforcements. A project covers the initial consultation, planning, structural engineering, project management and turnkey interior finishing, so you are not hiring a separate firm for each stage.
If you are planning a rear, side or kitchen extension, see our services or get in touch through the Kovacs Construction website to talk through your house and what you want from it.
FAQ
How much does a house extension cost in London?
The HOA’s 2026 guide puts single-storey extensions in London at £2,300 to £3,400 per m², above its UK range of £1,800 to £3,000. A 20 m² extension at those rates would be roughly £46,000 to £68,000 before VAT and any kitchen or special finishes. An itemised quote on full drawings gives the real figure.
Is £40,000 enough for an extension?
It can cover a small single-storey extension outside London at the lower end of published rates, but in London it buys less space. At the HOA’s London range of £2,300 to £3,400 per m², £40,000 covers roughly 12 to 17 m² before VAT. Ask for a quote that shows what is and is not included.
Do I need an architect as well as a builder?
Not necessarily. An architect can guide the project from brief to site inspection, according to RIBA. A design and build contractor covers the design, structural work, approvals and construction under one contract, so you appoint one party instead of several.
Do I need planning permission for a house extension?
Sometimes not, if the work falls within permitted development, but building regulations approval is separate and applies to almost all extensions. Listed buildings and conservation areas need consent from the local authority. Start with the Planning Portal and confirm with your builder before signing a contract.
