Basement
conversions
in Hampstead Garden Suburb, NW11
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Before any basement design in the Suburb we check the house type, the neighbours and the road outside, because the Trust does not normally approve basements under semi-detached, terraced or small houses.
The Suburb was designed without basements, and the joint guidance says they are not part of its established character, including the scale of each house's accommodation and its balance with the neighbours.
That makes the first question for any Suburb basement whether the house is one where a basement could be acceptable at all, before any thought of rooms or finishes.
Source: Hampstead Garden Suburb Trust and Barnet Council: Conservation Area Design Guidance (2010)
Which Suburb Houses Can Take a Basement
The Trust's 2016 supplementary guidance says basements will not normally be approved under semi-detached or terraced houses, because of the risk of differential movement.
Basements will not normally be approved under small cottages or modest family houses either, to protect the range of house types and sizes.
In closes and narrow roads, a basement may not be approved because of the disruption to other households during the work.
We check the house type, its neighbours and the road outside before commissioning any survey, because those three answers often settle the question.
Source: Hampstead Garden Suburb Trust: Supplementary Basement Guidance (approved 16 February 2016)
Depth, Footprint and the Garden Above
Where a basement is acceptable in principle, the Trust asks for no more than one storey, about three metres deep, within the original footprint of the house.
The 2010 joint guidance limits any extension under the garden to larger houses with extensive gardens, and the Trust asks for at least one metre of permeable soil over any part of a basement beneath a garden.
The Trust asks for no lightwells, pavement lights, grilles or railings at the front or a visible side, and the joint guidance says lightwells or skylights must not be visible from the street or from neighbours.
Sources: Hampstead Garden Suburb Trust: Supplementary Basement Guidance (approved 16 February 2016), Hampstead Garden Suburb Trust and Barnet Council: Conservation Area Design Guidance (2010)
Clay, Water and the Houses Next Door
The Trust records that in adjoining or linked houses on shallow foundations on clay, a basement can cause differential movement, leaving the houses either side open to cracking.
The Trust and Barnet may require a hydrogeological report, a full engineer's report on structural stability and a method statement for removing excavated soil.
The basement must not displace groundwater or surface water so as to cause flooding, and should include a sustainable drainage scheme and adequate headroom, daylight and ventilation.
Under the party wall rules set out on GOV.UK, digging below and near the foundation level of a neighbour's property means telling them first.
Sources: Hampstead Garden Suburb Trust: Supplementary Basement Guidance (approved 16 February 2016), Hampstead Garden Suburb Trust and Barnet Council: Conservation Area Design Guidance (2010), GOV.UK: Party walls and building work
Barnet's Basement Policy
Barnet Local Plan Policy CDH06 says basements must not damage valuable trees or roots on the site or next door, must suit the ground and groundwater conditions, and must be subordinate in size to the building.
The policy asks for rooms of adequate size and shape with acceptable natural light and ventilation, and habitable basement rooms must meet the London Plan Policy D6 minimum ceiling heights.
Underpinning needs the Trust's consent on its own, in addition to any planning permission.
Sources: Barnet Local Plan 2021-2036 (adopted 4 March 2025), Hampstead Garden Suburb Trust and Barnet Council: Conservation Area Design Guidance (2010)
Listed Houses and Basements
Basements will not normally be acceptable in listed buildings because of their effect on the historic internal layout.
With around 500 individually listed buildings in the Suburb, checking the list entry comes first.
Where a basement is ruled out, the alternatives are usually above ground: a reorganised ground floor, a modest extension within the guidance, or a garage conversion, which needs the Trust's consent.
Sources: Hampstead Garden Suburb Trust and Barnet Council: Conservation Area Design Guidance (2010), Hampstead Garden Suburb Trust - Your Property
Basement projects we have completed nearby
Questions about a Suburb basement
Can I add a basement under a semi-detached house in the Suburb?
Not normally. The Trust's guidance says basements will not normally be approved under semi-detached or terraced houses because of the risk of differential movement.
How deep can a Suburb basement be?
Where it is acceptable at all, no more than one storey, about three metres deep, within the original footprint of the house.
Can a basement extend under the garden?
The joint guidance allows it only for larger houses with extensive gardens, with at least one metre of permeable soil above, while the Trust's 2016 guidance asks that a basement stays within the original footprint of the house.
Which reports will be needed?
The Trust and Barnet may require a hydrogeological report, a full engineer's report on structural stability and a method statement for removing excavated material.
Are basements possible in listed houses?
Not normally, because of their effect on the historic internal layout.
Sources: Hampstead Garden Suburb Trust: Supplementary Basement Guidance (approved 16 February 2016), Hampstead Garden Suburb Trust and Barnet Council: Conservation Area Design Guidance (2010)
Sources
- Hampstead Garden Suburb Trust and Barnet Council: Conservation Area Design Guidance (2010)
- Hampstead Garden Suburb Trust: Supplementary Basement Guidance (approved 16 February 2016)
- GOV.UK: Party walls and building work
- Barnet Local Plan 2021-2036 (adopted 4 March 2025)
- Hampstead Garden Suburb Trust - Your Property
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Hampstead Garden Suburb history & architecture
Background on the streets, buildings and conservation history of Hampstead Garden Suburb: The Influence of Architects on Hampstead Garden Suburb · The Red Brick Suburbs of Hampstead Garden Suburb · Converting Lofts in Hampstead Garden Suburb · The Atmosphere of Hampstead Garden Suburb Roads · Planning Rules for Hampstead Garden Suburb Facades · The Early History of Hampstead Garden Suburb
Completed renovation project examples
