How to Cost Your Extension

With any home development project, one of the biggest considerations is the cost. Most of us will have a budget of some kind to work with but how do you know what you can do with this budget? How do you select the right kind of extension for your home but also your budget?

Self-build extension costs

When looking into the cost of self-build extensions there are lots of variables that make it difficult to say exactly what a project will cost without specifics. But there are some general ideas to help and factors to consider to get some ideas.

For example, some of the variables that might affect the cost of the project include:

• What the extension is going to be used for
• What size extension you want
• The location of your home (the most expensive locations can put up the price by as much as 30%)
• The type of soil around the property (as it affects foundations)
• How much glazing you want in the extension

Experts say that a straightforward extension will cost around £1000–£2000 per square metre. For a higher specification, this cost will increase and can be around 40% more for the very highest specification available.

Experts say that a straightforward extension will cost around £1,000–£2,000 per square metre.

Build or self-build?

Another factor in the cost of the extension is whether you have someone build it for you or go down the self-build route. Self-build has become more popular and there are also many variations that allow you to handle the aspects of the project you have the skills for while handing off other areas to contractors. Some people choose to project manage or do just some jobs themselves such as interior decorating.

There are four common routes for the extension:

1. DIY – build it mostly yourself, saving around 30% on labour while using contractors for specialist areas
2. Self-managed with contractors – hiring contractors directly with minimal DIY
3. Main & sub-contractors – a package company supplies to a certain stage, subcontractors finish on-site
4. Main contractor – builder manages the full project with minimal homeowner involvement

Other costs to consider

The more storeys the extension has, the more it will cost. A single-storey, basic spec extension might cost £1000–1700 per m² while a top-spec could cost £1900–2100 per m². A two-storey extension does not double the price per m², as foundations and roofing are only built once.

A common budgeting mistake is forgetting about VAT. Most materials and labour will be subject to the standard 20% VAT rate. Some projects can receive VAT relief, including:

• Converting a dwelling and changing number of units (5%)
• Working on a listed building (0%)
• Homes unoccupied for 2+ years (5%)

To gain these benefits, you must use a VAT-registered builder.

Quote versus estimate

When getting prices for an extension, it’s crucial to understand the difference between a quote and an estimate.

An estimate is an educated guess based on experience and provided information. It is not legally binding.

A quotation is a fixed price. A written quote should itemise work, include a breakdown of costs, clarify if VAT is included, and list any caveats. A quotation is typically legally binding.

Shop around

No matter what route you take for your extension or what type of extension you want, it is a good idea to shop around and talk to different experts. From oak framed extensions to two-storey brick extensions, there are many options and speaking with experts helps you find the right one.

Last updated: February 14, 2025