Forming an accurate budget for your project

Building project costs vary greatly depending on size, finish and choices for the interior fit out. That said – the average price for a standard extension stable and predictable. We have broken down the costs for each stage of a common configuration below.

Your specific budget will be estimated and agreed at time of contract, based on the calculations made by your architect. Once underway, your budget will be closely monitored.

However – there is always the possibility of unforeseen problems. For instance, the Covid pandemic caused sudden materials shortages and obstacles are occasionally found when digging foundations. Any consequential impacts on cost are addressed and adjustments calculated as soon as they are identified. For this reason, it is wise to add a contingency when calculating your overall extension cost.

Case study: 30m2 flat roof extension

30m2 is the largest extension you can build without planning permission, making it the most popular choice. The guide prices below detail the typical elements (including typical glazing options) in single storey flat roof extension. We have not included the cost of kitchen units since you may wish to buy these separately.

Stage 1: Super structure

This first stage results in a bare but weather-proof shell

Cost Covers
Ground works (foundations, drainage)
Build walls
Fit roof
Fit windows

Stage 2: Steelwork and first fix

Stage 2 is where services are added and the structures integrated

Cost Covers
£8,000 Demolition of walls to unify old and new
Fitting of structural steel where walls removed
First fix carpentry / electrics / plumbing

Stage 3: Second fix to plaster finish

Interior spaces are plastered and services terminated for use

Cost Covers
£7,000 Dry lining & plastering
Second fix carpentry / electrics & plumbing

Cost for completed build: £55,000 + VAT

Stage 4: Fitting and finishing

The finishing stage makes your space ready to use

Cost Covers
£20,000 Kitchen supply & install
£2,000 Flooring supply & fit
£1,000 Painting & decorating

Optional: luxury extras

The costs above will result in a great new space ready to use. For those with larger budgets, luxury extras can be integrated which will maximise your experience.

Cost Covers
£4,000 Upgrade to quartz worktops
£5,000 Aluminium Bi-fold doors

Don’t forget: Professional fees

Don’t forget to factor in the fees you’ll need to ensure your design meets regulatory and safety standards

Cost Covers
£1,000 Architect fee for planning drawings
£500 Structural engineer fee
£1,000 Building control / inspections