Single Storey Extension Costs in 2026: An Honest Guide

What you should expect to pay

For a shell-and-core build to a good standard, with plastering, wiring, plumbing first and second fix, windows, doors and a finished roof, most Aylesbury projects land in these brackets. Kitchens, bathroom suites, flooring and decorating usually sit outside a builder's quote unless you ask for them to be included.

The per-square-metre rate falls as the extension gets bigger. A 12 sq m infill costs more per metre than a 30 sq m rear extension, because foundations, scaffolding, welfare and site setup are largely fixed costs spread over less floor area.

• Small rear extension, around 12 to 15 sq m: £38,000 to £50,000
• Typical rear extension, around 20 to 25 sq m: £55,000 to £75,000
• Large wraparound, 30 to 40 sq m: £80,000 to £120,000
• Add £6,000 to £12,000 for a mid-range kitchen supply and fit
• Add £4,000 to £9,000 for large sliding or bifold doors, depending on span and specification

What actually moves the price

Groundworks are the single biggest unknown. Parts of Aylesbury Vale sit on clay, and shrinkable clay near mature trees can mean foundations at 1.8m to 2.5m rather than 900mm. Every extra 500mm of depth across a footprint adds concrete, muck away and labour, and it is entirely possible for foundations alone to swing a project by £5,000 to £10,000. Older properties in villages such as Wendover, Haddenham or Weston Turville can also throw up rubble footings or wells that only appear once the ground is open.

Drainage is the second. If a public sewer runs under or within three metres of the proposed extension, you will need a build over agreement with Thames Water, which takes time and money. Diverting a run or building a new inspection chamber is common and rarely quoted for accurately at the sketch stage.

After that, it is mostly specification. Structural steel to open up a rear wall, roof lanterns, underfloor heating, a flat roof in single ply versus felt, and the glazing you choose will each move things by thousands. Two extensions of identical size can differ by £25,000 on finish alone.

The costs people forget to budget for

Homeowners often budget the build and get caught out by everything around it. Set aside a realistic allowance for the following before you commit.

• Architectural drawings and planning application: roughly £1,500 to £4,000 through Buckinghamshire Council, depending on whether you need full planning or permitted development
• Structural engineer's calculations for steels and foundations: £500 to £1,200
• Building control, either through the council or an approved inspector: £700 to £1,300
• Party wall surveyor if you are within three metres of a neighbour's foundations: £1,000 to £2,000 per adjoining owner
• Skips and waste: several loads on a typical job, and clay spoil is heavy
• Contingency: 10 to 15 percent, and treat it as spent until the job is finished

Reading a quote properly

A one page quote with a single number is not a quote, it is a guess. A proper one breaks the job into stages, states what is excluded, names the specification for windows, insulation and roof covering, and sets out a payment schedule tied to completed work rather than dates. If a quote is 20 percent below the others, find out what has been left out before assuming you have found a bargain.

Ask for a provisional sum where something genuinely cannot be known, such as foundation depth, and get the rate agreed in advance for each extra cubic metre. That way a surprise in the ground becomes an arithmetic problem rather than an argument. It is also worth agreeing in writing who covers the cost of delays caused by the build over agreement or a party wall award, since both routinely run to six weeks.