Extension costs in London range from roughly £25,000 for a garage conversion to over £180,000 for a double storey addition, and the biggest driver of where a project lands isn’t location, it’s project type. Here’s the real breakdown, based on current market data and figures from Extension Architecture’s own extension cost calculator, cross checked against actual project costs the practice has delivered across London and Surrey.
Cost by Extension Type
| Project Type | Typical Cost Range | Cost Per m² |
| Garage conversion | £25,000 to £45,000 | Varies by scope |
| Loft conversion | £45,000 to £95,000 | £1,500 to £2,500 |
| Single storey rear extension | £75,000 to £120,000 | £1,800 to £2,500 |
| Kitchen extension | £80,000 to £115,000 | Varies by specification |
| Double storey extension | £130,000 to £180,000 | Around £2,200 |
Double storey extensions offer better value per square metre than they might appear at first glance, since the roof and foundation costs get shared across two floors rather than one, which is why the per m² figure drops slightly compared to a single storey build of comparable finish.
What Actually Moves the Number Within Each Range
- Bifold or sliding door width. Wider glazed openings cost more in structural support and hardware, scaling noticeably from a 2m opening up to 6m or 8m.
- Number of skylights. Each additional skylight adds both material and installation cost.
- Kitchen specification. A basic fit-out costs considerably less than a premium specification with integrated appliances and bespoke joinery.
- Roof style. A flat roof generally costs less than a dual pitch or crown roof design.
- Property type and existing structure. Older properties, particularly Victorian and Edwardian terraces, often need floor or roof reinforcement that adds to the base cost before any finishes are considered.
Costs Beyond the Build Itself
The headline figures above cover construction, but several smaller costs sit alongside them and catch people out when they’re not budgeted in from the start. A standard householder planning application costs £258, though many single storey projects avoid this fee entirely under permitted development. Building regulations approval, required regardless of planning route, typically adds £850 to £1,200 for inspections and certification. Where the extension sits near a shared boundary, Party Wall Act notices and any resulting surveyor agreement generally add £700 to £1,600 per adjoining owner. None of these individually change the overall budget dramatically, but together they can add several thousand pounds that a headline construction figure alone doesn’t capture, which is why a full quote should always itemise them separately rather than folding them into one round number.
Why the Same Extension Type Costs More in Some Areas
Location affects cost primarily through two channels, site access and the complexity of the surrounding build environment, rather than a simple postcode multiplier. A tight urban terrace with limited access for machinery and materials tends to cost more per square metre than a project with open access and space to work, since materials and equipment often have to be moved manually rather than delivered directly to the build zone.
Oxshott architects illustrate the other end of this spectrum. Oxshott’s large detached homes on generous plots, often inspired by Palladian and neo-classical design principles, tend to involve larger scale extensions with bespoke architectural detailing, stone columns, formal courtyards, ancillary buildings, which pushes total project costs well above the standard ranges above, even though the underlying per m² build cost follows similar principles. The difference comes down to scale and specification rather than the fundamentals of the build itself.
Using a Cost Calculator Properly
- It typically excludes interior specifications, finishes, and bespoke joinery
- It doesn’t account for unique structural alterations specific to a property
- It can’t factor in site access constraints until a survey is done
- Postcode-based estimates capture regional variation but not street-level factors like conservation area requirements
The honest use case is narrowing down a realistic budget range before committing time to a full feasibility assessment, not as a substitute for one.
What This Means for Value
Most well planned extensions add 15 to 25% to a property’s value, though this depends heavily on how the extension integrates with the rest of the house and the local market ceiling for that property type. A kitchen extension that opens up the ground floor tends to have the most consistent impact on how a property shows to potential buyers, since it directly affects how the most used room in the house functions day to day. A loft conversion that adds a genuine extra bedroom, particularly one with an en suite, also tends to perform well against its cost, since bedroom count is one of the clearest drivers of asking price in most London postcodes.
Getting an Accurate Number for Your Property
Generic ranges and calculator estimates are a starting point, not a quote. The only way to know what a specific property will actually cost is a proper feasibility assessment that accounts for site access, structural condition, and design complexity together.
With 17 years of experience and over 1,800 planning approvals across London and Surrey, Extension Architecture manages design, planning, structural engineering, and building regulation drawings under one roof, with an in-house structural engineer, a setup only around 1 in 20 firms genuinely offer.
