If you're planning a house extension in Barnet or North London, understanding the likely costs upfront is essential for budgeting. As builders who specialise in extensions across Barnet, Finchley, Hampstead, Mill Hill and the surrounding areas, here's our honest 2026 pricing guide.
House Extension Costs in Barnet — Overview (2026)
Single-Storey Rear Extension: £35,000 – £65,000. Single-Storey Side Return Extension: £40,000 – £70,000. Wrap-Around Extension (rear + side): £60,000 – £100,000. Double-Storey Extension: £70,000 – £120,000. Kitchen Extension (single-storey): £40,000 – £75,000.
These figures include all construction work, glazing, roofing, electrics, plumbing and basic finishes. Kitchen and bathroom fitting, flooring and decoration are typically additional.
What Affects Extension Costs?
Key cost factors include: roof type (pitched costs more than flat; glazed lantern roofs add £5,000–£15,000); glazing specification (bi-fold or sliding doors add £8,000–£20,000); structural complexity; and finish specification (standard versus premium can add £15,000–£30,000).
Does an Extension Add Value?
Yes — substantially. In North London, a well-designed single-storey rear extension typically adds 10–20% to the value of a property. For properties in Barnet, Hampstead and Finchley, this regularly represents £80,000–£200,000 in added value.
What's Included in Our Extension Quote?
Every Construction Collective quotation includes: structural engineer drawings, Building Regulations application and inspections, foundations and groundworks, masonry construction, roof structure and covering, windows and external doors, first fix electrics and plumbing, and plastering.
Get an Accurate Quote
Every project is different, which is why we offer a free site visit to every homeowner in Barnet and North London. You can also use our Online Price Guide to get an instant budget estimate in under 60 seconds. Call 020 8441 3144 or message us on WhatsApp.