House
extensions
in Belsize Park, NW3
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Hampstead Chartered Surveyors & Building ConsultancyRear and side extensions for Belsize villas and red-brick houses, drawn to Camden's Belsize statement and its Home Improvements tests, with the backs seen from the next street and the gaps between houses in mind.
Camden's Belsize statement sets the tone for any addition at the back: keep it as unobtrusive as it can be, and in most cases to a single storey.
We look at the house from its garden, from its neighbours' gardens and from the street behind before drawing an extension, because in Belsize all three views can matter.
Source: Camden: Belsize Conservation Area Statement (agreed 20 November 2002)
What the Belsize Statement Says About Extensions
An extension should harmonise with the house's original form and with the historic pattern of extensions in its terrace or group.
Infilling yards and the spaces between buildings at the rear will generally be unacceptable.
Where a side extension would not lose an important gap, the statement asks for it to be single storey and set back from the front building line.
Camden's Belsize design guide covers only Article 4 works and sends readers to Camden's website for rear extensions, lightwells, dormers and garden sheds.
Sources: Camden: Belsize Conservation Area Statement (agreed 20 November 2002), Camden: Belsize Conservation Area Design Guide (updated June 2026)
Backs Seen From the Next Street
Because Belsize backs are often visible from adjoining streets, the statement will not accept an extension that breaks the uniform rear of an unspoilt terrace or group.
Its example is the view from Belsize Park Gardens across to the rear of houses on Belsize Park and Belsize Square, where extensions are prominent.
Camden's guide says mature trees, green gardens and glimpses of gardens between houses give the buildings their setting.
Sources: Camden: Belsize Conservation Area Statement (agreed 20 November 2002), Camden: Belsize Conservation Area Design Guide (updated June 2026)
Camden's Home Improvements Tests
Camden's guidance wants rear extensions subordinate to the house in location, form, footprint, scale, proportions and detailing, in sympathetic materials that respect the original design.
Daylight to neighbours is tested too: Camden applies its 45-degree and 25-degree rules, or accepts a daylight study that follows BRE guidance.
It should leave a reasonably sized garden, and avoid excessive light spill onto neighbours, including flats above where a house has been divided.
For a green roof, Camden prefers a biodiverse one with 100mm of substrate over sedum.
Source: Camden Planning Guidance: Home Improvements (January 2021)
Trees, Gardens and Foundations
Applications must map every tree within 10 metres of the work, below ground as well as above, the statement says.
In a conservation area, Camden must be given advance notice of work to any tree whose trunk is over 75mm in diameter, measured 1.5 metres up.
Clay that swells and shrinks needs deeper foundations, the Planning Portal warns, and nearby trees can take them well below the depth first expected.
Approved Document H's building-over-sewers provisions apply to building or extending within 3 metres of the centreline of an existing drain or sewer on the sewerage company's records.
Sources: Camden: Belsize Conservation Area Statement (agreed 20 November 2002), Camden Council: Conservation areas, Planning Portal: Extensions, Building Regulations: Foundations, GOV.UK: Approved Document H, Drainage and waste disposal (2015 edition)
Permitted Development in a Conservation Area
Government permitted development guidance excludes, in a conservation area, any cladding of the house, any extension beyond a side wall and any rear extension over one storey.
Householder permitted development does not apply to a building containing flats or to a flat within one.
Even where a proposal is permitted development, building regulations approval and Party Wall Act requirements still apply.
Source: MHCLG: Permitted development rights for householders, Technical Guidance (September 2019)
Openings, Beams and Party Walls
Most extensions need building regulations approval, the Planning Portal says.
Forming a new or wider opening in the back wall means supporting the wall above on new beams with proper bearings; a steel beam normally needs fire protection.
Building at the boundary, or excavating close to and below a neighbour's foundations, means serving notice on them first, GOV.UK says.
Sources: Planning Portal: Is building regulations approval needed for an extension?, Planning Portal: Extensions, Building Regulations: Structural Opening, GOV.UK: Party walls and building work
Garden-level additions nearby
Questions about extending a Belsize Park house
How big can a rear extension be in Belsize?
The statement asks for it to be as unobtrusive as possible and, in most cases, no more than one storey, in keeping with the pattern of extensions in the terrace or group.
Can I build a side extension?
Possibly, where it would not lose an important gap: the statement then asks for a single storey set back from the front building line.
Is a rear extension permitted development in Belsize?
Within narrow limits only: in a conservation area, permitted development does not cover cladding, extensions beyond a side wall, or rear extensions taller than one storey.
Will my neighbours' daylight be tested?
Yes. Camden asks for its 45-degree and 25-degree tests to be met, or for a daylight test showing compliance with BRE guidance.
Do trees affect the design?
Yes. Applications must identify every tree within 10 metres, and Camden requires notice before work to trees above a set size in a conservation area.
Sources: Camden: Belsize Conservation Area Statement (agreed 20 November 2002), MHCLG: Permitted development rights for householders, Technical Guidance (September 2019), Camden Planning Guidance: Home Improvements (January 2021), Camden Council: Conservation areas
Sources
- Camden: Belsize Conservation Area Statement (agreed 20 November 2002)
- Camden: Belsize Conservation Area Design Guide (updated June 2026)
- Camden Planning Guidance: Home Improvements (January 2021)
- Camden Council: Conservation areas
- Planning Portal: Extensions, Building Regulations: Foundations
- GOV.UK: Approved Document H, Drainage and waste disposal (2015 edition)
- MHCLG: Permitted development rights for householders, Technical Guidance (September 2019)
- Planning Portal: Is building regulations approval needed for an extension?
- Planning Portal: Extensions, Building Regulations: Structural Opening
- GOV.UK: Party walls and building work
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