Build cost calculatorPrime London 2026

Your build cost, in a minute.

This is the same rate card our quantity surveyor works from. Pick the type of work and enter the floor area. You get the construction range for that work. Fees, contingency and VAT sit outside it, so we show them separately.

See the 2026 ranges
Since 2010Building prime London homes
BCISRates behind every figure
7 daysA budget estimate, typically

Open the tool

Tell us where to send it.

Your name and email, and the tool opens below. We ask so your estimate reaches you in writing.

Your estimate, and nothing passed to anyone else. Your details go to the studio and our email system. Read the privacy note.

We send your estimate, then occasional prime London cost updates. One click unsubscribes.

The calculator is open below. We have your details, so the studio can follow it up.

Open plan living, dining and kitchen under a long rooflight.
Chelsea · living, dining and kitchen opened up under one rooflightCompleted work, photograph

By the square footThe rate card

The rate card, by the square foot.

A rate moves with the specification, the access and the state of the building. Pick the band that matches the work.

These are the calculator’s own bands, built from one file. Each runs from our lowest specification to our highest. They price construction only.

  1. 01 Full renovation

    £155 to £420 per sq ft

  2. 02 Loft conversion

    £215 to £425 per sq ft

  3. 03 Side return

    £215 to £425 per sq ft

  4. 04 Rear extension

    £235 to £460 per sq ft

  5. 05 Double storey extension

    £235 to £460 per sq ft

  6. 06 Wraparound extension

    £235 to £460 per sq ft

  7. 07 New build

    £330 to £660 per sq ft

  8. 08 Basement

    £450 to £780 per sq ft

By the jobEight types

Eight jobs, priced.

Whole jobs, as we build them. Construction only, at 2026 prime London rates.

Single storey rear addition

Rear kitchen extension

£160,000 to £250,000

Opening up the rear for a larger kitchen and dining space. Linked to the garden.

Infill the side passage

Side return extension

£140,000 to £220,000

Widening the ground floor of a Victorian or Edwardian house for light and flow.

Rear and side expansion

Wraparound extension

£200,000 to £320,000

Rear plus side return, for a much larger and brighter ground floor.

Ground and first floor addition

Two-storey extension

£220,000 to £400,000

Ground floor living space, plus bedroom accommodation above it.

Classic rear roof extension

Main dormer loft

£90,000 to £140,000

A new bedroom suite or office level, a rear dormer and an improved stair.

Roof level rebuilt for volume

Mansard loft

£140,000 to £220,000

Substantial roof reconstruction. Planning context and heritage detailing both apply.

Upgrade existing lower ground space

Existing basement conversion

£180,000 to £300,000

A gym, a cinema room, a guest suite or a family snug.

Create new space below ground

New basement excavation

£500,000 to £900,000

From £500,000, and above £900,000 on larger schemes. Leisure, wellness, staff or family spaces where site conditions allow.

Read these with the ranges

A range is the build. A project is more.

A build range prices construction. A whole project carries more than that, and those are real costs.

So we show them on their own line, rather than inside the rate.

See the 12 stages

Professional fees, statutory costs, party wall, contingency and VAT are separate.

  • Professional fees, set per project by our quantity surveyor
  • Statutory costs, including building control and planning
  • Party wall awards, where a neighbour is affected
  • Contingency, because an existing building carries unknowns
  • VAT, treated separately

Ranges are guide prices, not fixed quotations. Our quantity surveyor sets the fees and the contingency per project. Your property is priced properly in a cost plan.

After the number

A number today. A cost plan next.

Send us the address and the scope. A budget estimate is typically with you within seven days.

01

We read the scope

Nick reads it himself. You get a free call, and an honest view of the work.

02

A budget estimate

Our own quantity surveyor prices the brief. It is typically with you within seven days.

03

A cost plan

Built line by line from the drawings. Provisional sums are named, not buried.

Your estimate is a guide price. We will tell you what needs to happen next.

Start a conversationGet the 2026 cost guide

From the journal402 notes, free to read

Where these numbers come from.

  1. 4 min read

    Side return or rear extension? Cost and how to choose

    Side return or rear extension for a London terrace? What each one is, what moves the cost of each, and how a design and build contractor chooses.

  2. 6 min read

    What a main contractor owes an architect at tender

    Six things a tender return should give an architect. Bill of quantities, method statements, shop drawing sign off and subcontractor detail.

  3. 2 min read

    Full house renovation cost London: why the band is so wide

    Why a London full renovation is quoted as a band. What puts a project at each end. How to use the figure before drawings exist.

Every note on cost