Basement
conversions
in Highgate, N6
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Highgate's hill is sand and gravel on top and London Clay below, with springs between, so a basement here is an engineering question before it is a design.
Camden's appraisal describes sand and gravel on the highest ground and London Clay on the lower slopes, with many springs, streams and ponds.
The neighbourhood plan ties its concern about basement conversions to the sensitive geology of the Hampstead Highgate Ridge.
Sources: Camden: Highgate Conservation Area Appraisal and Management Strategy (adopted October 2007), Highgate Neighbourhood Plan (adopted by Haringey in July 2017 and Camden in September 2017)
The neighbourhood plan's Policy DH7
Where a basement falls outside permitted development and there is evidence of drainage or flooding problems, DH7 supports it only if it meets the policy's requirements, beginning with no harm to the stability of neighbouring properties.
It expects at least a metre of permeable soil over any part of a basement beneath a garden, and a pumped device or other flood protection against sewage backflow.
An application must satisfy DH7 and the relevant borough's own policy and guidance together.
Source: Highgate Neighbourhood Plan (adopted by Haringey in July 2017 and Camden in September 2017)
On the Camden side
Since June 2017 a Camden-wide Article 4 Direction has meant that no basement or lightwell can be dug under permitted development; each one needs an application.
Policy A5 holds a Camden basement to a single storey, under no more than 50% of any garden and smaller than 1.5 times the footprint of the house, and the impact study must keep the predicted damage next door within Burland category 1.
Camden wants that technical work signed by a chartered engineer or chartered geologist in the right professional body.
Sources: Camden Planning Guidance: Basements (January 2021), Camden Local Plan 2017, Camden Council: Basement developments and planning applications
On the Haringey side
Haringey's Policy DM18 permits new or extended basements only where they protect structural stability with regard to local geology, avoid flood risk and protect the property's setting, neighbours and the environment.
Beneath a garden, DM18 also protects valued trees and asks for soil deep enough for landscaping like the neighbours'.
Haringey requires a Basement Impact Assessment with the application and will not accept habitable basement rooms in flood-prone areas without a reasonable escape route.
Source: Haringey: Development Management DPD (adopted July 2017)
Light wells
Haringey weighs a light well against the building's character, the area's appearance and whether more than half of the front garden would be lost.
Camden points out that in a shallow front garden a lightwell tends to swallow most of the space, and it resists them in streets where front lightwells are not already the pattern.
A rear lightwell or sunken court is often the better way to bring daylight to a Highgate basement.
Sources: Haringey: Development Management DPD (adopted July 2017), Camden Planning Guidance: Basements (January 2021)
Groundwater, ponds and trees
The plan records concern that basements reduce the groundwater that feeds the Highgate and Hampstead ponds.
With houses being rebuilt on larger footprints over extensive basements, it says protecting mature trees is essential to limit the impact on local hydrology and the water table.
Camden's basement guidance explains that springs tend to rise at the junctions between the hill's layers: Bagshot Formation over Claygate Member, and Claygate Member over London Clay.
Sources: Highgate Neighbourhood Plan (adopted by Haringey in July 2017 and Camden in September 2017), Camden Planning Guidance: Basements (January 2021)
Neighbours and construction
The neighbourhood forum recommends basement work only between 8am and 6pm on weekdays, unless exceptional circumstances apply.
Camden generally requires a Construction Management Plan for basement development.
Neighbours get formal notice under the Party Wall Act wherever the dig goes below and close to their foundations.
We share the monitoring plan with neighbours before excavation and report readings through the dig.
Sources: Highgate Neighbourhood Plan (adopted by Haringey in July 2017 and Camden in September 2017), Camden Local Plan 2017, GOV.UK: Party walls and building work
What a Basement Impact Assessment covers
Camden's guidance sets out five stages: screening, scoping, site investigation and study, impact assessment, and review and decision.
Each assessment should include a plain summary that someone without technical knowledge can follow.
Risk to neighbours is described on the Burland Scale, whose categories reflect how easily visible damage could be repaired.
We commission the site investigation early, because boreholes and groundwater readings on Highgate's hill can change the design before anything is drawn in detail.
Source: Camden Planning Guidance: Basements (January 2021)
Keeping a Highgate basement dry
A basement here is designed for water from the start. The structure holds back the ground, a drained cavity or membrane collects any water that gets through, and pumps carry it away to the drains.
The neighbourhood plan asks for a pumped device or other protection against sewage backing up into the basement.
Pumps are fitted in pairs with an alarm and a battery backup, and we leave access to them for servicing.
Source: Highgate Neighbourhood Plan (adopted by Haringey in July 2017 and Camden in September 2017)
A Highgate basement we have built
Questions about basements in Highgate
Why are basements assessed so closely in Highgate?
The neighbourhood plan says the area's geology, topography and hydrology have led to well-documented problems with residential basements.
Does my basement need planning permission?
On the Camden side, yes, because of the borough's 2017 Article 4 Direction.
What does Haringey require?
Its Policy DM18 tests stability, flood risk, appearance, neighbours and the environment, and it asks for a Basement Impact Assessment.
How much soil must be left over a basement under the garden?
The Highgate plan expects at least a metre of permeable soil above any part beneath a garden.
Will I need a pump?
The plan asks every basement to include a positive pumped device or similar protection against sewage backflow.
What does a basement conversion in Highgate cost?
Pricing: quoted after a feasibility visit.
Sources: Highgate Neighbourhood Plan (adopted by Haringey in July 2017 and Camden in September 2017), Camden Planning Guidance: Basements (January 2021), Haringey: Development Management DPD (adopted July 2017)
Sources
- Camden: Highgate Conservation Area Appraisal and Management Strategy (adopted October 2007)
- Highgate Neighbourhood Plan (adopted by Haringey in July 2017 and Camden in September 2017)
- Camden Planning Guidance: Basements (January 2021)
- Camden Local Plan 2017
- Camden Council: Basement developments and planning applications
- Haringey: Development Management DPD (adopted July 2017)
- GOV.UK: Party walls and building work
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