Guide · 13 October 2026

How to plan a budget for building work (without guessing)

You'll find plenty of "cost per square metre" figures online. We don't publish one, because two London extensions of the same size can differ hugely depending on ground conditions, access, the steelwork needed and the finish you choose.

What actually drives the cost

• Structure: steel beams, foundations and any underpinning.
• Access: a mid-terrace with no rear access means everything goes through the house.
• Fees: architect, structural engineer, planning or Lawful Development Certificate, building control and possibly a party wall surveyor.
• Finish: glazing, flooring, kitchen and lighting can be a large share of the total.

Steps to a reliable figure

1. Get measured drawings and, where needed, structural calculations first.
2. Ask for an itemised quote so you can compare like with like.
3. Check what's excluded (skips, scaffolding, making good, VAT).
4. Keep a contingency for things that can't be seen until walls are opened up.
5. Agree stage payments linked to finished work, not dates.

Tell us about your project through the enquiry form and we'll explain what we'd need to give you a proper quote.

Official fee information: GOV.UK – Planning permission and GOV.UK – Building regulations approval.