It is one of the most common stories in London home improvement. A family saves for years and decides to improve rather than move. They work with an architect, ask for the best the house can offer — a generous extension, a beautiful kitchen, perhaps a new shower room — and planning permission is granted. Then the builders' quotes arrive, and they are far above the budget they had in mind.
If this has happened to you, nobody has done anything wrong. Planning drawings show what is allowed to be built, not what it will cost to build. The cost conversation simply hasn't happened yet. This guide explains where the gap comes from and how to close it calmly.
Why online figures and averages are rarely right
A quick search, or asking an AI assistant, will often produce a single neat number — "a 30 m² extension costs about £75,000". The figure isn't wrong as an average; it just can't know your house. Two extensions of the same size can differ by more than £100,000 because of things that only a site visit and full drawings reveal:
- Access. Is there side access, or does every barrow of soil and every steel beam go through the house? Will a crane or grab lorry be needed?
- Ground and foundations. London clay, nearby trees and drains can mean deeper trench foundations — or screw piles.
- Drainage. Building over or diverting a sewer needs agreement and adds work.
- Structure. Opening the back of the house or taking out the side-return wall needs steel beams, posts and pad foundations.
- Living at home. Working around a family is slower and needs more protection than working in an empty house.
- The level of finish. This is the biggest one, and it deserves its own section.
"Extension" can mean three very different things
When people compare prices they are often comparing different scopes. For the same 30 m² rear extension, indicative London prices in 2026 look like this (excluding VAT):
| Scope | What you get | Indicative range |
|---|---|---|
| Shell only | Foundations, walls, steelwork, roof and glazing — weathertight, ready for trades | £55,000–75,000 |
| Shell, first fix & plaster | Plus electrics and plumbing run in, insulation, plastered walls and screeded floor | £75,000–100,000 |
| Fully finished | Plus kitchen, flooring, underfloor heating, lighting, decoration — ready to live in | £120,000–180,000+ |
Add a new shower room, a utility or laundry, landscaping the garden afterwards, or premium glazing, and the total moves again. None of this is visible on a planning drawing.
The costs that are easy to forget
- VAT at 20%. A £200,000 budget that has to include VAT covers about £167,000 of building work.
- Professional fees: structural engineer, building control, party wall surveyors.
- A contingency of around 10% for what is found when walls and floors are opened.
- Things outside the build: furniture, curtains and blinds, moving out for a few weeks.
Reading quotes: like for like
When quotes differ widely, the reason is usually what each one includes. A short quote with a single total may leave items as "provisional sums" or exclusions that are priced later. An itemised quote shows every line, so you can see exactly what is included and what each decision costs. When you compare, ask every contractor the same questions: what is excluded, which items are provisional, and is VAT included?
What to do if your budget and the quote don't match
Say your preferred quote is £260,000 and your budget is £200,000. There are honest ways to close the gap without compromising what matters:
Share your real budget early
Including VAT, fees and contingency. It isn't a negotiating weakness — it lets the designer and builder aim at the right target from the start.
Get a cost check before planning, not after
A builder can price a concept design in days. Adjusting a drawing costs far less than redesigning after approval.
Simplify the structure
Fewer steel beams, a simpler roof and standard glazing sizes can save tens of thousands without changing how the space feels.
Choose the finish level room by room
Premium where you touch and look every day — the kitchen worktop, the taps, the floor — and essential elsewhere.
Phase the project
Build the shell, first fix and plaster now, and fit the kitchen or finish a second bathroom when funds allow. The structure is done once and done properly.
Never cut what you can't see
Foundations, waterproofing, insulation and drainage are the wrong places to save. They are expensive to fix later.
How we handle it
We ask about budget in the first conversation, because it shapes every recommendation we make. If a design and a budget don't match, we say so early and show options — usually essential, premium and luxury — with an itemised price for each. If you already have planning drawings, send them to us for a free cost check before you go to tender.
For a first range, try our cost calculator, and see the kitchen extension cost guide for a worked example.
Frequently asked questions
Why is my extension quote so much higher than I expected?
Planning drawings show what can be built, not what it costs. Access, foundations, drainage, steelwork, the level of finish, VAT and fees all change the price, so online averages are rarely accurate for a specific house.
What is the difference between shell only and a fully finished extension?
Shell only is the weathertight structure: foundations, walls, steel, roof and glazing. First fix and plaster adds services, insulation, plaster and screed. Fully finished adds the kitchen, flooring, heating, lighting and decoration.
What can I do if quotes are over my budget?
Share your full budget early, get a cost check before planning, simplify the structure, choose the finish level room by room, or phase the project — but don't cut foundations, waterproofing, insulation or drainage.
Want a price for your project?
Use our calculator for an instant range, or book a free survey for a fixed, itemised quote.
