About this page: Home Extensions London is a free enquiry service run by Web Designer Limited (trading as Zoib). We pass enquiries to independent extension builders and design-and-build firms; we don’t carry out the work, and we can’t guarantee that every enquiry is taken up. Figures below are quoted from the named sources with their dates; they are not our prices. More about this site.
Two ways to procure an extension
Most London extensions are bought in one of two ways. Neither is right for every house.
- Architect or designer first, then builders tender. You commission drawings (and, where needed, a planning application and structural calculations), then send the same package to several builders. Quotes are easier to compare because everyone is pricing the same thing.
- Design-and-build. One firm handles drawings, approvals and construction. It can be simpler to manage, but it is harder to compare prices, because each firm designs and prices its own scheme.
Whichever route you take, permitted development rights or planning permission, Building Regulations and the Party Wall etc Act 1996 are separate regimes. The government’s permitted development technical guidance says plainly that permitted development rights do not remove the need for building regulations approval or Party Wall consents.
What to have ready before you ask for quotes
- The consent route. Know whether you are relying on permitted development, prior approval for a larger rear extension, or a householder planning application. Our page on extension rules in London explains the limits. The Planning Portal fee schedule lists £548 for a householder application in England from 1 April 2026 and £249 for prior approval of a larger home extension.
- Drawings and, ideally, structural calculations. Builders can only price steelwork, foundations and openings properly if they know what is needed.
- A finishes list. Flooring, glazing type, rooflights, kitchen (supplied by you or by the builder), heating and lighting. Unspecified items turn into provisional sums.
- Party Wall timing. GOV.UK explains that notice must be given to adjoining owners for certain work; a builder’s start date should leave room for that process.
Checks to run on any extension builder
- Who you are contracting with. If it is a limited company, look it up on Companies House: check the registered name, status, filing history and directors match the paperwork.
- Insurance. Ask for current public liability insurance and, for larger jobs, contractor’s all-risks cover. Check the policy dates and the insured name.
- Scheme membership. If a firm says it is TrustMark registered, check it on the TrustMark homeowner site rather than relying on a logo. Do the same for any trade association.
- Recent, similar work. Ask for addresses or contacts for comparable London extensions, ideally ones finished in the last two years, and ask those owners about programme, communication and snagging.
- Who signs off Building Regulations. Ask whether the builder uses your local authority building control or a registered building control approver, and who applies. GOV.UK explains when approval is needed.
- Electrics and gas. Notifiable electrical work should be certified by an electrician registered with an authorised competent person scheme or signed off by building control; any gas work must be done by a Gas Safe registered engineer.
Citizens Advice also has a useful checklist before you get building work done, and GOV.UK publishes advice on using trader recommendation sites.
Comparing quotes like for like
Ask every builder to price the same drawings and the same specification, and to show the following separately:
| Item | What to ask |
|---|---|
| Preliminaries | Scaffolding, skips, welfare, site protection and any licences. |
| Demolition and groundworks | Foundation type and depth assumed; what happens if ground conditions or drains differ. |
| Structure | Steel beams, padstones and temporary works, priced to the engineer’s calculations. |
| Roof and glazing | Rooflights, doors and screens by make or performance, with lead times. |
| Services | Drainage, heating, electrics and certification. |
| Finishes | Plastering, flooring, decoration; whether the kitchen is supplied and fitted. |
| Provisional sums | Each one listed with its amount and what it covers. |
| VAT | Whether 20% VAT is included or added. |
For a sense of scale, Checkatrade’s house extension cost guide gives about £1,800 to £3,000 per m² for a full extension, and HomeOwners Alliance’s extension cost guide suggests a single-storey extension in London may cost about £2,300 to £3,400 per m². Those are national and regional ballparks published by those sites, not quotes. Our page on kitchen extension costs goes into more detail.
Contract, payments and sign-off
- Use a written contract that names the parties, the drawings and specification, the price, the programme, how variations are agreed and priced, and how disputes are handled.
- Link staged payments to completed stages of work (for example, foundations signed off, roof on and watertight), not to dates. Be wary of large up-front deposits.
- Agree a retention or a snagging period before final payment.
- At the end, you should receive the Building Regulations completion certificate, electrical certificates and any gas paperwork. Keep them; you will need them when you sell.
- Under section 49 of the Consumer Rights Act 2015, a trader must perform a service with reasonable care and skill.
If you would like us to pass your enquiry to an independent extension builder who covers your area, use the enquiry form. We don’t build extensions ourselves, and the builder you speak to is an independent business.
Questions people ask
How many quotes should I get for an extension?
There is no legal number, but asking at least three builders to price the same drawings and specification makes it far easier to spot gaps and outliers. A much cheaper quote often means something has been left out or turned into a provisional sum.
Should I use a design-and-build firm or an architect?
Both can work. An architect-led route makes quotes easier to compare because every builder prices the same package. Design-and-build can be simpler to manage, but you are comparing different designs as well as different prices.
Does the builder deal with Party Wall notices?
Sometimes, but the legal duty to serve notice under the Party Wall etc Act 1996 sits with the building owner. GOV.UK explains when you need to tell your neighbours, and many owners appoint a party wall surveyor.
Who arranges Building Regulations approval?
It can be you, your designer or your builder, but agree it in writing. Either your local authority building control or a registered building control approver can be used. Ask who will apply and who will book the inspections.
Sources
- GOV.UK – Permitted development rights for householders: technical guidance
- Planning Portal – Fees for planning applications in England (from 1 April 2026)
- GOV.UK – Party walls and building work
- GOV.UK – Building regulations approval
- GOV.UK – Competent person schemes
- TrustMark – homeowner information
- Companies House – find and update company information
- Citizens Advice – before you get building work done
- GOV.UK – What to consider when using trader recommendation sites
- Consumer Rights Act 2015, section 49
- Checkatrade – house extension cost guide
- HomeOwners Alliance – house extension costs
Sources checked on 20 October 2026. General information only, not legal, planning, structural or financial advice.
Related pages
- Kitchen extension cost
Published 2026 figures for kitchen extension costs, what quotes usually include and leave out, and the London premium. - Loft conversions in London
Velux, dormer, hip-to-gable and mansard lofts: permitted development limits, Building Regulations, Party Wall and London issues. - Loft conversion cost
2026 published loft conversion costs by type and per m², the cost breakdown, hidden extras and the London premium.