Behind the numbers
How the extension cost calculator works
The Extension Builder prices an extension the way a builder does: the floor area multiplied by a rate, then the options added on. Every step of the tool feeds that sum, and the price updates the moment you change one.
- Size. The floor area you add, in square metres. For a double-storey extension it is the area across both floors.
- Finish level. Standard, premium or high-end sets the base rate: £2,800, £3,400 or £4,200 per m² for a single-storey rear extension.
- Extension type. Each type adjusts that rate. A side return costs a little more per m², with a party wall and tight access to work around. A double-storey costs less, because two floors share one foundation and one roof.
- Roof and glazing. Each option adds a set amount, shown on its card before you pick it.
- A minimum for small builds. Steels, drainage and setting up the site don’t shrink with the room, so each type has a floor price.
The answer comes as a range, from 10% below the central figure to 15% above it. It leans upwards on purpose: what turns up once work starts, such as deeper foundations or an old drain in the way, adds cost far more often than it saves it.
What the price includes, and what it leaves out
The guide price is the build: groundwork and foundations, the structure and any steels, walls, roof, glazing, plastering and a standard finish. It leaves out four things you should budget for separately:
- VAT. Prices are before VAT. Most extension work is standard-rated, so add 20%.
- Professional fees. Design, a structural engineer (around £600–£1,500), building control and any party wall surveyors. Allow 10–18% of the build.
- The kitchen. Units, worktops and appliances are typically £8,000–£30,000 on top, depending on spec.
- Basements. The calculator doesn’t price them. Basement work in London typically costs £4,500–£7,500+ per m²; see basement conversion costs.
Extension cost per m² in 2026
These are the rates the calculator uses, per square metre of new floor area, before VAT. The single-storey rates match the £2,800–£4,200 per m² range in our North London extension cost guide; the other types are adjusted from there.
| Type of build | Standard | Premium | High-end |
|---|---|---|---|
| Single-storey rear or kitchen extension | £2,800 | £3,400 | £4,200 |
| Side return | £2,940 | £3,570 | £4,410 |
| Wrap-around | £2,940 | £3,570 | £4,410 |
| Double-storey rear (across both floors) | £2,100 | £2,550 | £3,150 |
| Loft conversion | £2,660 | £3,230 | £3,990 |
| Garage conversion | £1,680 | £2,040 | £2,520 |
A loft comes in a little under a ground-floor extension because there are no foundations to dig, and a garage lower again because its walls and roof are already there. For lofts by type, from roof windows to a mansard, see loft conversion costs in London.
Worked examples: common extensions priced
Here is what the calculator gives for the projects people ask about most. Ground-floor examples include a roof lantern and bifold doors, the tool’s starting options; the loft is a hip-to-gable with a rear dormer and roof windows. Pick a row to load it into the calculator and change the details.
| Project | Size | Standard finish | Premium finish |
|---|---|---|---|
| 3m × 5m single-storey rear | 15 m² | £47,500–£61,000 | £56,000–£71,500 |
| 4m × 4m single-storey rear | 16 m² | £50,000–£64,000 | £59,000–£75,000 |
| 20 m² kitchen extension (4m × 5m) | 20 m² | £60,500–£77,000 | £71,000–£91,000 |
| Side return | 10 m² | £36,500–£46,500 | £42,000–£53,500 |
| Wrap-around | 32 m² | £94,500–£121,000 | £112,500–£144,000 |
| Double-storey rear (20 m² per floor) | 40 m² | £81,000–£103,500 | £97,000–£124,000 |
| Hip-to-gable loft with rear dormer | 24 m² | £63,000–£80,500 | £75,000–£96,000 |
| Single garage conversion | 14 m² | £21,000–£27,000 | £25,500–£33,000 |
Leave out the lantern and bifolds and the price drops: the same 20 m² kitchen extension with a plain flat roof and standard glazing comes out at £50,500–£64,500, against £60,500–£77,000 with them.
More on each project: 3m × 5m extension costs, single-storey extensions, 20 m² kitchen extension costs, side return extensions, kitchen extensions, two-storey extension costs, loft conversion costs, garage conversion costs. Orangeries and garden rooms aren’t in the calculator: see orangery costs and garden room costs.
What moves the price
Three of the calculator’s choices move the number most.
- Finish level. The biggest lever. The same 20 m² kitchen extension costs about 40% more at a high-end finish than at standard: £85,500–£109,000 against £60,500–£77,000.
- Glazing. Over standard doors and windows, bifold doors add £6,000, sliding panels £7,500 and Crittall-style screens £9,500. In a loft, a glazed dormer face adds £2,000 and a Juliet balcony £3,000.
- Roof. A roof lantern adds £5,000. A pitched roof adds about 8%, for the extra structure and the tiles to match the house. Turning a hipped loft into a hip-to-gable adds £6,000.
The rest only shows up when someone stands in your garden:
- Ground. Much of North London sits on clay, and nearby trees or drains can push foundations deeper.
- Party walls. On terraces and semis you will usually serve party wall notices. If neighbours appoint surveyors, budget £1,000–£2,500+ per neighbour; our party wall guide explains how it works.
- Access. A tight street with no side access means more labour, plus skip and scaffold licences.
- Structure. Opening the back of the house into one big room needs steel, and the wider the opening, the bigger the beam.
- Conservation areas. Matching materials and extra design work add cost, and some permitted development rights are lost. See extensions in conservation areas.
Planning: the likely route and what it costs
The calculator’s last step shows the planning route your design is likely to need. It works from typical plot widths, so treat it as a first read: we check your address against the council’s maps on the site visit.
| Route | Usually applies to | Council fee |
|---|---|---|
| Permitted development, confirmed by a lawful development certificate | Single-storey rears up to 3m deep (4m detached); side returns up to half the width of the house; most lofts within 40 m³ (terraces) or 50 m³ (semis and detached) | £274 |
| Prior approval (larger home extension) | Single-storey rears 3–6m deep (4–8m detached); neighbours are consulted | £249 |
| Householder planning application | Wrap-arounds, most double-storey extensions, larger side extensions and anything in a conservation area that falls outside permitted development | £548 |
Applying through the Planning Portal adds a £91.02 service charge, which you avoid by paying the council directly. The full breakdown is on our planning fees page. For whether you need permission at all, read the planning permission guide, and for why the certificate matters when you sell, the lawful development certificate guide.
From an online estimate to a real quote
The calculator gets you to a budget in a minute. A price you can build to takes a visit to the house. Book one and we bring the design you made here, then:
- measure the house and garden, and look at access, drains and the neighbours’ extensions;
- check your address for conservation areas and anything else that changes the planning route;
- talk through layout, glazing and finish, the choices that move the price most;
- follow up with an itemised quote, so you can see where every pound goes.
The visit is free and there is no obligation. To see finished work first, look through our projects, including the Hadley Wood house extension.