Extension Cost Guide

Extension Cost Guide

How much will your home extension cost? Price up your project and get your budget on track with these benchmark extension cost breakdowns

by Chris Bates

3rd July 2020


There are 3 steps in this guide

  1. Home Extension Step-by-Step Guide
  2. Do I Need Planning Permission for my Home Extension?
  3. Extension Cost Guide

If you’ve got empty garden space, then adding an extension can be a great way to improve your home and increase its value.

Prices for extension projects vary significantly depending on factors such as the size, number of storeys, specification and which part of the country you’re building in.

As a general rule, extension prices can start from as little as £1,000 per m2 for a straightforward conservatory.

A fairly basic single-storey addition that’s fully integrated into the main house might begin at around £1,200 per m2. You can expect this to rise to well over £2,500 per m2 for a high-end design.

To help you get a better idea of how much your home extension might cost, we’ve priced up three different types of extension built to a fairly standard quality level, using the Build It Estimating Service. For the purposes of easy comparison, all of these example projects are located in the Midlands.

Note that our benchmark extension costs do not include figures for elements such as kitchen cabinets, appliances or sanitary ware, as the prices for these can vary wildly according to your specification choices.

Conservatory extension cost guide

If you’re looking for a cost-effective route to increasing your home’s living space, then a conservatory could be the best way forward. Provided a scheme meets the relevant criteria, then it can be completed under permitted development rights – plus you may not need to go through the usual building control rigmarole, either.

A conservatory extension by Crystal Direct (illustrative purposes only; does not reflect the project costs shown here)

Our model conservatory, based on a design by Crystal Direct, will add 20m2 of space to an existing property. The zone will have to be separated from the main house using external-quality doors and must be heated independently, too, to avoid Building Regulations inspections. For this reason, it will be more of a garden room than an extension of the current living space.

Benchmark Conservatory Extension Costs

  • Project type: Conservatory extension
  • Floorspace added: 20m2
  • Build cost: £19,588
  • VAT: £3,918
  • Extension cost: £25,506
  • Extension cost per m2: £1,175
Elements Cost m2 Cost % Total Cost
Commencement £27 3% £531
Foundations & footings £137 14% £2,741
Oversite & slabbing £66 7% £1,326
Brickwork £60 6% £1.194
Conservatory £414 42% £8,282
Internal preparation £1 <1% £25
Structural openings £74 8% £1,478
First fix electrics & plumbing £34 4% £685
Plastering £120 12% £2,398
Second fix joinery, electrics & plumbing £33 3% £666
Internal decoration £13 1% £262
SUBTOTAL £979   £19,588
VAT @20% £196   £3,918
GRAND TOTAL £1,175   £23,506

The Build It Estimating Service prices the work at £23,506 including labour, plant, materials and VAT – and we should be able to complete the project in just seven weeks. The biggest outlay will be on the main conservatory structure, at £8,282 before VAT, and payment for this will be expected on delivery (around week three or four).

Single storey extension cost guide

Using the Build It Estimating Service, we’ve priced this 20m2 single-storey rear extension at £34,662 including labour, plant, materials and VAT.

The project features bifold doors and a roof lantern, and could be the perfect choice for creating an open-plan kitchen diner or a similar zone.

An illustration of our benchmark 20m2 flat roof extension

We expect the works to take around eight weeks – so slightly longer than adding a conservatory, plus we’re anticipating an extra cost of just over £11,000.

But the result will be more integrated into the existing house, with a shared heating system and no need to separate the new space with external quality doors.

Benchmark Single Storey Extension Costs

Elements Cost m2 Cost % Total Cost
Commencement £27 2% £531
Foundations & footings £137 9% £2,741
Oversite & slabbing £66 5% £1,326
Brickwork shell £302 21% £6,045
Roof structure £341 24% £6,825
Roof covering £79 6% £1,589
Internal preparation £1 <1% £25
Structural openings £74 5% £1,478
First fix electrics & plumbing £60 4% £1,199
Plastering £159 11% £3,172
Second fix joinery, electrics & plumbing £175 12% £3,494
Internal decoration £19 1% £371
External decoration £4 <1% £89
Subtotal £1,444   £28,885
VAT @20% £289   £5,777
Grand total £1,733   £34,662


A similar design with a pitched roof would come in slightly more expensive, at around £36,388 including VAT.

This is largely down to the extra costs involved in erecting and finishing a gable wall inside and out. In terms of the covering, while a flat roof structure would cost more to build, surfacing it is significantly cheaper – so the total price comes out around the same.

Double storey extension cost guide

Taking on a double-storey addition can give you maximum bang for your buck, adding considerable living space while minimising the potential impact on garden amenity.

An illustration of our 40m2 two-storey pitched roof extension

From a cost perspective, you can think of a two-storey extension as equivalent to stacking a couple of boxes on top of each other. There’s more wall than with a single storey addition, but the amount of roofing and foundations required will be very similar.

Benchmark Two Storey Extension Costs

Elements Cost m2 Cost % Total Cost
Commencement £57 4% £2,288
Foundations & footings £70 5% £2,790
Oversite & slabbing £34 3% £1,375
Brickwork shell £372 28% £14,862
First floor joists £22 2% £897
Roof structure £106 8% £4,4256
Roof covering £145 11% £5,798
Internal preparation £2 <1% £100
Structural openings £45 3% £1,797
First fix electrics & plumbing £116 9% £4,639
Plastering £115 12% £6,191
Second fix joinery, electrics & plumbing £155 12% £6,193
Internal decoration £33 3% £1,317
External decoration £4 <1% £142
Completion £1 <1% £26
Subtotal £1,317   £52,671
VAT @20% £263   £10,534
Grand total £1,580   £63,205


The Build It Estimating Service gives a total cost of £63,205 for our 40m2 double-storey extension. We’ll be on site for 13 weeks and it will be more difficult to live in the house during the works, so we may need to pay for rental accommodation.

At £1,580 per m2 of extra living space, this model double storey extension project offers better value for money than the flat roof single storey extension. However, the overall cost of a two storey extension is greater.

Basement extension cost guide

This benchmark project is a conversion and extension of an existing cellar to create a 33.9m2 zone suitable for a living area, small bathroom and store.

It involves lowering the floor and underpinning the walls to gain sufficient headroom, which swallows up a significant amount of the costs.

Labour is the single biggest cost, at over 60%, but it’s vital to use a specialist for at least the watertight shell phase – so there’s not a great deal of opportunity to save money with a DIY approach during this phase. While the cost per m2 is high, it could still represent great value in urban centres or where other types of extension aren’t viable.

Loft conversion cost guide

Our benchmark loft project is based on a straightforward conversion that creates 40m2 of internal floor area. The prices assume that the roof tiling does not need to be replaced, but do include scaffolding fees.

The rooflight conversion includes the use of seven Velux windows, inserted into the roof, at a cost of around £1,000 each for the labour and materials.

The dormer conversion features three dormer windows (pitched structures built out of the roof plane), with two at the rear and one at the front, plus a single rooflight. This will create extra head height and allows for the inclusion of standard windows (in this case, aluminium).