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
31st July 2018

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 sanitaryware, 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.

Crystal Direct Conservatory

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 Regs inspections. For this reason, it will be more of a garden room than an extension of the current living space.

Extension costs: Conservatory

ElementsCost m2Cost %Total Cost
Grand total£1,175£23,506
Commencement£273%£531
Foundations & footings£13714%£2,741
Oversite & slabbing£667%£1,326
Brickwork£606%£1.194
Conservatory£41442%£8,282
Internal preparation£1<1%£25
Structural openings£748%£1,478
First fix electrics & plumbing£344%£685
Plastering£12012%£2,398
Second fix joinery, electrics & plumbing£333%£666
Internal decoration£131%£262
Subtotal£979£19,588
VAT @20%£196£3,918

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.

Single Storey Extension illustration by HBXL

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.

Extension Costs: Single Storey

ElementsCost m2Cost %Total Cost
Grand total£1,733£34,662
Commencement£272%£531
Foundations & footings£1379%£2,741
Oversite & slabbing£665%£1,326
Brickwork shell£30221%£6,045
Roof structure£34124%£6,825
Roof covering£796%£1,589
Internal preparation£1<1%£25
Structural openings£745%£1,478
First fix electrics & plumbing£604%£1,199
Plastering£15911%£3,172
Second fix joinery, electrics & plumbing£17512%£3,494
Internal decoration£191%£371
External decoration£4<1%£89
Subtotal£1,444£28,885
VAT @20%£289£5,777

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.

Double Storey Extension illustration by HBXL

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.

Extension Costs: Double Storey

ElementsCost m2Cost %Total Cost
Grand total£1,580£63,205
Commencement£574%£2,288
Foundations & footings£705%£2,790
Oversite & slabbing£343%£1,375
Brickwork shell£37228%£14,862
First floor joists£222%£897
Roof structure£1068%£4,4256
Roof covering£14511%£5,798
Internal preparation£2<1%£100
Structural openings£453%£1,797
First fix electrics & plumbing£1169%£4,639
Plastering£11512%£6,191
Second fix joinery, electrics & plumbing£15512%£6,193
Internal decoration£333%£1,317
External decoration£4<1%£142
Completion£1<1%£26
Subtotal£1,317£52,671
VAT @20%£263£10,534

Hence at £1,580 per m2, this model project offers better value for money than the flat roof extension – albeit the overall outlay is greater.

The Build It Estimating Service gives a 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.

Main image: This project features theEDGE sliding doors by IDSystems

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