The preparing of the site and digging of foundations is essential to making sure the extension is built safely and with structural integrity. Though costs for this phase are difficult to separate out, if there are complications at this stage, such as having to move drains or deal with tree roots, it can slow down the process and increase the cost.

This is where the main building takes place, building the basic structure with either brick and block or a timber frame. A roof is built, and interior elements such as plasterboard and a subfloor are added to ensure a watertight structure. Costs here are typically related to the size of the build. Nowadays, both brick and blockwork or timber frame extensions are at a similar price point, so while they may have various advantages and drawbacks in certain situations, the main building material choice should not be too much of a cost consideration for the average extension job.

Fitting out the extension with features like skirting boards, doors, electrical sockets and switches, as well as necessities like radiators. The costs of all of these will depend on whether you want to purchase high-end materials or not. Sliding or bifold doors can cost thousands of pounds per metre, while simple French doors will cost much less.

There is no upper limit to what you can spend when finishing the extension – every individual element will have a wildly different cost, from cheap carpet that is a few pounds per square metre, to imported marble tiles that will costs hundreds of pounds to cover the same space. Costs will also increase significantly if you are adding a new kitchen or bathroom to the extended space. As a benchmark, a new kitchen could cost between £7,000 and £25,000, while a bathroom could cost between £3,000 and £10,000.