House
extensions
in Hampstead, NW3

In a conservation area, three things fall outside permitted development entirely: cladding any part of the house, any extension beyond a side wall, and a rear extension that adds more than one storey.

A flat is not a house for these rules either. Government guidance excludes flats, and buildings that contain them, from its definition of a dwellinghouse.

Check the planning history as well. A council can withdraw permitted development rights with an Article 4 Direction, or may already have done so by condition on an earlier permission for the house. Hampstead's own Direction concentrates on fronts and street-facing sides.

If you believe a project is permitted development, you can apply to Camden for a Lawful Development Certificate to be certain before you build.

Sources: MHCLG: Permitted development rights for householders, Technical Guidance (September 2019), Camden Council: Hampstead Conservation Area Design Guide (updated June 2026)

Camden's Home Improvements guidance describes a well-judged rear extension: subordinate to the house in position, form, footprint, scale, proportion and detail; built from sympathetic materials; and respectful of original features such as projecting bays, cornices and chimney stacks.

It must also leave a reasonably sized garden and meet the daylight checks in Camden's Amenity guidance, the 45 degree and 25 degree tests, or show compliance with BRE guidance through a daylight test.

On style the guidance is open. An inventive design can sometimes solve an awkward site, while elsewhere an extension that matches the house fits better.

Side additions are more visible, so the rules tighten. Camden expects them to be single storey in the ordinary case, and the 2001 statement resists infilling a gap between buildings where the gap matters or a symmetrical composition would suffer; an acceptable side extension should be set back from the front building line.

Inside its boundary, the Hampstead Neighbourhood Plan, adopted on 21 July 2025, supports extensions that are subservient to the original footprint and mass of the house, add positively to local character and improve biodiversity. It also expects applicants to show that any loss of light to neighbours is acceptable under Camden's guidance.

Sources: Camden Planning Guidance: Home Improvements (January 2021), Camden: Hampstead Conservation Area Statement (agreed 31 October 2001), Camden Council: Hampstead Neighbourhood Forum, Hampstead Neighbourhood Plan 2025-2040 (adopted by Camden)

Policy D2 of Camden's Local Plan protects trees and garden spaces that contribute to a conservation area's character or form a setting for its buildings.

Work to a tree in a conservation area needs advance notice to Camden where the trunk is more than 75mm across at 1.5 metres above ground.

Foundations and drainage runs can damage roots, so we fix the footprint around the trees first and commission an arboricultural survey wherever the extension comes close to one.

If new foundations will be dug below and close to the level of a neighbour's foundations, or the wall is built on the boundary itself, GOV.UK says the neighbour has to be told.

Sources: Camden Local Plan 2017, Camden Council: Conservation areas, GOV.UK: Party walls and building work

Camden's guidance suggests a preliminary assessment before designing: look at the existing rear elevation and any earlier additions, how visible the back of the house is from gardens and the street, the pattern of neighbouring development, and rear extensions next door built with permission or as permitted development.

Camden's homeowner pre-application advice has two levels. A single-storey rear extension can sit in the first; two-storey rear extensions, or more than one extension, belong in the second.

In a conservation area the application must include a heritage statement describing the significance of the buildings affected and justifying any harm. Where the Neighbourhood Plan applies and a design and access statement is needed, it has to explain how the extension conserves or enhances the relevant Hampstead character area.

• Measured survey of the house, the garden levels and the neighbouring windows that the daylight checks will test.
• Design options drawn in plan and section, with the party wall and tree constraints shown, so the decision on depth and height is made on evidence.
• Structural design and party wall notices prepared together, so the neighbours see the same drawings the engineers are working from.
• Kitchen layout and joinery coordinated with the shell; see our kitchen renovation page for how the two are combined.
• A site plan for deliveries, spoil and protection of the neighbours' boundaries, agreed before work starts.

Sources: Camden Planning Guidance: Home Improvements (January 2021), Camden Council: Pre-planning application advice for homeowners, Camden Council: Conservation areas, Hampstead Neighbourhood Plan 2025-2040 (adopted by Camden)