Basement Conversions in South Hampstead, NW6
South Hampstead has seen many basement schemes, some large enough to swallow gardens, so Camden reads each new one against the garden it would dig into as well as the house above.
Camden's appraisal records a spate of new and enlarged basements here, dug with lightwells front and back, some of them, on Aberdare Gardens for example, so large that they spill into the gardens.
We start a basement enquiry with the garden survey and our engineer's desk study, and only then sketch any rooms.
Source: Camden: South Hampstead Conservation Area Character Appraisal and Management Strategy (February 2011)
Camden Local Plan Policy A5
Under Policy A5 of Camden's 2017 Local Plan, a basement is permitted only once the council is content that neighbours, the ground, groundwater, the street's character and the house itself will not suffer.
The policy holds a basement to a single storey that is not dug beneath an existing one, covers at most half of each garden, stays under one and a half times the house's footprint and keeps back from the boundaries with neighbours.
The Basement Impact Assessment has to keep the predicted damage to neighbouring buildings within Burland category 1, very slight.
Camden has required an application for every basement and lightwell since 1 June 2017, when its Article 4 direction took basements out of permitted development.
Sources: Camden Local Plan 2017, Camden Planning Guidance: Basements (January 2021)
What the South Hampstead appraisal adds
The appraisal sees a modest basement, kept inside the original footprint and going down a single full storey of about 3 metres, as often the most suitable way to gain space below ground, if trees, water and the garden are unaffected.
A basement that takes up a whole front or back garden is not acceptable; most of the garden must stay unexcavated, with wide undeveloped strips along the boundaries for trees and planting.
Lightwells, sunken courtyards, skylights and stepped planting can spoil what garden is left, and the permitted size depends on the site and the local pattern of built and open land.
Camden will resist a basement that is too big for its host building or neighbours where the conservation area would be harmed.
It also flags how splitting a building into more units can bring new excavation and the loss of front gardens and walls.
Source: Camden: South Hampstead Conservation Area Character Appraisal and Management Strategy (February 2011)
Lightwells, railings and front gardens
A new lightwell can loosen a house's tie to the street and use up garden, the railings round it add clutter, and rooflights planted in a front garden can let light escape at night.
Camden's basement guidance warns that a lightwell would probably take up much or all of a shallow front garden, and points to side or rear lightwells kept away from the boundary.
Front gardens have already suffered: in 2008/9 Camden found almost a quarter of boundary walls removed and over two fifths of front gardens given over to parking.
Sources: Camden: South Hampstead Conservation Area Character Appraisal and Management Strategy (February 2011), Camden Planning Guidance: Basements (January 2021)
Surface water and the ground beneath NW6
Camden's basement guidance says modelling puts parts of South Hampstead among the areas at higher risk of surface water flooding.
Where flooding is a risk, Camden refuses basements that would hold bedrooms or other habitable space.
The gardens themselves matter here, since Camden credits them with reducing flood risk.
Any proposal to create or enlarge a basement has to include a detailed technical statement on its effect on drainage, flood risk, groundwater and the stability of the structure.
Sources: Camden Planning Guidance: Basements (January 2021), Camden Local Plan 2017, Camden: South Hampstead Conservation Area Character Appraisal and Management Strategy (February 2011), Camden Council: Basement developments and planning applications
Houses with a lower floor already
Belsize Road's terraces were built with basements, and some detached houses on Cleve Road and Compayne Gardens stand on raised basements.
With Policy A5 forbidding a second basement under an existing one, upgrading the lower floor a house already has is usually the realistic route.
Bringing an old lower floor up to standard still involves waterproofing, a fresh slab, insulation and more daylight, and we survey it with the same care as a new excavation.
Sources: Camden: South Hampstead Conservation Area Character Appraisal and Management Strategy (February 2011), Camden Local Plan 2017
Trees, roots and the arboricultural report
If trees grow on the plot or beside it, including those in the street, Camden wants an arboricultural report submitted with the application.
South Hampstead's tree cover runs mostly as continuous bands across the back gardens.
Where excavation extends beyond the building, Camden looks for a metre or more of soil over the roof of the basement so planting can grow.
Work to any garden tree with a trunk over 75mm across needs six weeks' notice to Camden.
Sources: Camden: South Hampstead Conservation Area Character Appraisal and Management Strategy (February 2011), Camden Planning Guidance: Basements (January 2021)
The Basement Impact Assessment
The assessment starts with screening and scoping, moves through site investigation and study to the impact assessment itself, and finishes with review.
Its technical chapters come from a chartered engineer or chartered geologist, with a summary in plain language for neighbours and other readers.
Camden must also be satisfied that the house and its neighbours will stay stable through demolition and construction.
We commission the boreholes and groundwater monitoring the assessment asks for, and warn the neighbours before the rig arrives.
Sources: Camden Planning Guidance: Basements (January 2021), Camden Council: Basement developments and planning applications, Camden: South Hampstead Conservation Area Character Appraisal and Management Strategy (February 2011)
Running a basement site in South Hampstead
A basement in Camden normally comes with a construction management plan, and in a conservation area the council expects one.
Digging and breaking out are noisy operations, so without Camden's authorisation they stop at 6pm on weekdays and 1pm on Saturdays and do not start before 8am.
These are quiet streets with few through routes and one-way traffic, the appraisal notes; we plan spoil lorry routes around them before work starts.
Sources: Camden Local Plan 2017, Camden Planning Guidance: Basements (January 2021), Camden Council: noisy building works, Camden: South Hampstead Conservation Area Character Appraisal and Management Strategy (February 2011)
Questions about South Hampstead basements
How deep can a South Hampstead basement go?
One storey: Policy A5 allows no more, and the appraisal favours about 3 metres within the original footprint.
Can the basement run under the garden?
Partly, under half of each garden at most, with most of the garden left unexcavated.
Do I always need to apply for a basement in Camden?
Yes; the 2017 Article 4 direction means even a lightwell needs permission.
Will a front lightwell be accepted?
Rarely: Camden says front lightwells can harm the street and a shallow front garden, and prefers side or rear ones.
Is my street at risk of surface water flooding?
Camden names parts of South Hampstead as higher risk and refuses habitable basement rooms in flood-prone areas, so the assessment checks your address.
Who signs off the technical work in the impact assessment?
Camden wants a chartered engineer or chartered geologist, suitably qualified, to write it.
Sources: Camden Local Plan 2017, Camden: South Hampstead Conservation Area Character Appraisal and Management Strategy (February 2011), Camden Planning Guidance: Basements (January 2021), Camden Council: Basement developments and planning applications
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