How to budget for a home extension in London

A realistic budget protects the design you want. These are the things that shape the cost of an extension, and how to plan for them.

What drives the cost

• Size and complexity – floor area matters, but so do structural openings, steelwork and roof design.
• Site access – terraced streets with limited parking and through-house access take more time and planning.
• Ground conditions and drainage – foundations and existing drains can change the scope below ground.
• Finishes – glazing, kitchens, flooring and joinery often make the biggest difference between two similar projects.
• Fees and approvals – design, structural engineering, planning, Building Control and any party wall costs.

Plan for a contingency

Older London homes often reveal surprises once work begins. Setting aside a contingency, commonly around 10–15% of the build cost, keeps the project moving if something unexpected turns up.

Why design and build helps

When the same team designs and builds, costs are checked as the design develops, not after the drawings are finished. Share your plans with us for an estimate based on your home.