How to Budget for a House Extension in London
Title: How to Budget for a House Extension in London (2026 Guide) Target keyword: budget house extension London
Budgeting for a house extension is one of the most common challenges London homeowners face. Most people start with a figure in mind — only to discover midway through the process that their initial number was incomplete. This guide walks you through how to build a realistic budget from scratch, so there are no nasty surprises.
Start with the Build Cost
The largest single cost in any extension project is the build itself. In London, typical contractor rates in 2026 run between £2,500 and £4,500 per square metre for a standard single-storey rear extension, depending on specification, borough, and structural complexity.
A 20m² extension at mid-range spec (say £3,200/m²) would have a build budget of around £64,000 before professional fees, compliance costs, or finishes.
Key variables that push build cost higher: - Structural steel requirements (bifold doors, large openings, RSJs) - Drainage rerouting or sewer build-over agreements with Thames Water - Party wall considerations if building near or on a shared boundary - Conservation area or listed building requirements, which affect materials
Add Professional Fees
Many homeowners budget for build costs only — and then get caught out by fees. A realistic professional fees budget for a single-storey extension includes:
Architect or designer: Typically 8–15% of build cost for full service (concept through to construction drawings). On a £64,000 build, this is £5,120–£9,600.
Structural engineer: Most extensions with openings or steel beams require structural engineer input. Budget £800–£2,500 depending on complexity.
Party wall surveyor: If a neighbour appoints their own surveyor, you pay both fees. Budget £1,500–£3,500 per appointed surveyor.
Budget for Planning and Compliance
If your extension requires planning permission (anything beyond permitted development thresholds), add: - Planning application fee: £258 for a householder application (as of December 2023) - Planning drawings/heritage statement: Already covered in architect fees if your designer prepares them
If you're going the permitted development route, you still need Building Regulations approval. Budget: - Building Regulations full plans application: £600–£1,200 depending on local authority - Building Control officer visits during build: Usually included in application fee
Don't Forget Finishes
Most contractor quotes exclude second-fix materials — flooring, tiles, kitchen units, bathroom fittings, light fittings, internal doors. These are often client-supplied. Budget separately: - Basic spec: £5,000–£12,000 for a kitchen extension - Mid-range spec: £12,000–£25,000 - High-spec: £25,000+
At RCB Design & Build, we make this clear from the outset. Our standard position is that all second-fix materials are client-supplied unless specifically agreed otherwise.
Add a Contingency
Budget a minimum 10–15% contingency on top of your total. On complex projects or older properties, 15–20% is prudent. Contingencies cover: - Unforeseen structural issues revealed during groundworks - Soil contamination or drainage problems - Changes to design during build - Material price movements on long projects
A Realistic Budget Example
For a 20m² single-storey rear extension with bifold doors in South London, a realistic all-in budget might look like:
| Item | Budget |
|---|---|
| Build cost | £64,000 |
| Architect (12%) | £7,680 |
| Structural engineer | £1,500 |
| Planning fee | £258 |
| Building regs | £900 |
| Party wall (contingency) | £2,000 |
| Finishes (client-supplied) | £15,000 |
| Contingency (12%) | £8,900 |
| Total | ~£100,000 |
This is a guide — your numbers will differ. But the point is that a £64,000 build cost can become a £100,000 project by the time all legitimate costs are accounted for.
The Value of a Pre-Construction Review
The most expensive mistakes happen when a project starts without proper cost planning. At RCB, our pre-construction process clarifies scope, design intent, compliance requirements, and a realistic cost before you commit. This means the figure you see in a proposal is grounded in real information — not a ballpark that grows later.
If you're planning an extension in London and want a structured, honest cost assessment, call us on 07359 872594 or visit rcbgroup.co.uk to start the process.