House extensions in Northwood: check the rules first, then build with confidence

The right extension depends on your house and plot. We build single-storey rear extensions, side and wraparound extensions, double-storey additions and open-plan kitchen and living spaces with bi-fold doors. On larger detached homes there is often room for a bigger rear extension; on semis, a well-planned wraparound or side-and-rear extension can transform how the ground floor works.

Northwood sits within the London Borough of Hillingdon, which has 31 designated conservation areas, and Northwood Frithwood is one of them. If your home is in a conservation area, or has an Article 4 direction, permitted development rights are restricted and you are likely to need planning permission for work that would otherwise be allowed.

Outside those areas, these permitted development limits commonly apply to houses in England:

• Single-storey rear extensions: up to 3 metres deep on semi-detached and terraced houses and 4 metres on detached houses, or up to 6 and 8 metres through prior approval.
• Side extensions: single storey only, no wider than half the width of the original house, and no higher than 4 metres.
• Multi-storey rear extensions: no more than 3 metres beyond the rear wall and at least 7 metres from the rear boundary.
• Materials must be of similar appearance to the existing house, and height limits apply near boundaries.

The safest first step is to check your address on Hillingdon Council's planning pages, and we will do this with you before we quote. If your project needs a Lawful Development Certificate or planning application, we work with architects and planning consultants and coordinate structural engineers and building control.

Source note: Planning Portal (extensions, planning permission); Hillingdon Council (conservation areas); planning.data.gov.uk (Northwood Frithwood conservation area).

Where planning conditions or your own taste call for an extension that looks original to the house, we carefully match brickwork, roof lines and windows so the new space reads as part of the property.

We do not publish one-size-fits-all prices because ground conditions, structure, glazing and finish change the figure a lot. As a guide, published 2026 London figures put single-storey extension construction at roughly £3,000 to £5,000 per square metre, before VAT and professional fees. Your quote is written, itemised and fixed, so you can see what is included.

Source note: Architecture for London, "How Much Does a House Extension Cost in London? 2026 Update".

1. 1Free consultation and site visit.
2. 2Written fixed-price quote.
3. 3Plans and approvals: architect, structural engineer, building control, planning where needed, and party wall notices if you share a wall.
4. 4The build, with one dedicated project manager, weekly progress updates and quality control inspections.
5. 5Handover. Snagging walk-through and completion paperwork.

Also explore house extensions in Uxbridge, loft conversions in Uxbridge, and home renovations in West London.