No. 01
Ground conditions & foundations
Clay soils, nearby trees and old drains can mean deeper or specially designed foundations. Trial holes and an engineer’s design settle this early.
Ask about this for ground conditions & foundationsExtension budgets explained, London-wide
What Will Your London Extension Cost?
Online price-per-square-metre figures rarely survive contact with a real house. This guide explains the things that actually move the number. When you’re ready, we can pass your enquiry to an independent builder for a line-by-line quote.
What moves the number
The biggest drivers of extension cost
Size matters, but it is rarely the whole story. These are the items that usually make two similar-looking extensions cost very different amounts.
No. 02
Steelwork & structural openings
Knocking through the back wall usually needs steel beams and padstones. Wide, clear openings with no posts need larger sections and more engineering.
Ask about this for steelwork & structural openingsNo. 03
Glazing, roof lights & doors
Large sliding or bifold doors and roof lanterns are often among the most expensive single items. Specification and size make a big difference.
Ask about this for glazing, roof lights & doorsNo. 04
Roof type
A simple flat roof generally costs less than a pitched or vaulted roof with tiles, but insulation and drainage still need to meet Building Regulations.
Ask about this for roof typeNo. 05
Kitchens, floors & finishes
The kitchen, flooring and underfloor heating can rival the cost of the shell itself. Ask for them to be priced separately so you can see where the money goes.
Ask about this for kitchens, floors & finishesNo. 06
Fees & approvals
Drawings, structural calculations, Building Control, and where needed a party wall surveyor and planning application all sit alongside the build price.
Ask about this for fees & approvalsWhy this approach
Pricing you can actually compare
A clear breakdown makes it easier to compare builders and to make decisions.
Like-for-like comparisons
Separate lines for each stage let you compare one builder’s quote with another properly.
Real measurements
Figures should be based on your house, not an average of other people’s projects.
Straight answers on savings
Ask which changes save real money and which barely move the total.
Programme alongside price
Ask how long each stage is likely to take, as well as what it costs.
Living through the build
Ask what disruption to expect and how the kitchen move will be handled.
Check insurance first
Builders work independently of this site, so ask for copies of their insurance certificates before a start date is agreed.
Areas covered
Extension pricing across London
Send an extension enquiry from any London borough and we’ll look for an independent builder who covers your area.
Costs are shaped by things that vary across the capital, such as access for materials, parking and skip permits, and how much space there is around the house.
Whichever borough you live in, a reliable quote is built from a visit to your property.
Any measure-and-price visit is arranged directly between you and the builder.
Write to us
Ask for an itemised extension quote
Describe the extension you have in mind. We’ll try to pass it to an independent builder who can visit, measure and price it.
Swap guesswork for a real figure
Send a free enquiry and ask for a line-by-line price for your extension.