Common basement policy restrictions in London boroughs

If you are planning a basement in London, the governing document is the borough's adopted local plan, not national policy and not the London Plan. A cluster of boroughs adopted specific basement policies and supplementary guidance after a run of large subterranean schemes in the 2010s, and while the numbers differ between authorities, the structure of those policies is remarkably consistent. Most of them restrict depth, cap the proportion of the garden that can be built under, control light wells on street frontages and require a technical assessment of ground and water conditions before permission is granted.

Read the policy before you draw anything. A basement designed to the limits of one borough's policy will often breach the next borough's, and the cost of redesign after a pre-application meeting is considerably higher than the cost of twenty minutes with the adopted plan.

Why boroughs adopted their own policies

National policy says very little about basements. What it says about flood risk, amenity, heritage and design applies, but there is no national limit on how deep you may dig under your own garden. Boroughs filled that gap because the planning issues that basements generate are local and cumulative: construction traffic in narrow streets, damage claims between neighbours, loss of garden soil depth and tree root space, altered groundwater flow across a terrace, and the sheer duration of the works.

The policies that followed tend to be written as a set of limits rather than as a judgement, because limits are easier to apply consistently and easier to defend at appeal. That is useful to an applicant. A policy with numbers in it tells you whether your scheme is acceptable in principle before you commission anything.

Depth and storey restrictions

The most common restriction is to a single storey below ground. Where a policy is drafted that way, a two level basement is contrary to the plan whatever its merits, and the only route is a case for a departure, which on a householder scheme is rarely worth pursuing.

Depth is sometimes controlled separately from storey count, by reference to a maximum excavation depth below existing ground level or below the lowest existing floor. Watch for the relationship between the two. A single storey basement with a 3.5m floor to ceiling height may exceed a depth limit written for a normal domestic storey.

Some policies also restrict basements under the existing building footprint differently from those extending beyond it. Under the footprint, the issue is usually structural and the policy is permissive. Beyond the footprint, under a garden or a lightwell, the issues are drainage, soil depth and trees, and the policy tightens.

• Check whether the limit is on storeys, on absolute depth, or on both
• Check from what level the depth is measured, because existing ground level and existing floor level can differ by a metre in a raised ground floor terrace
• Check whether the policy treats a basement under the footprint differently from one beyond it
• Check whether a swimming pool, plant room or cinema counts as a storey

Footprint, garden coverage and soil depth

Garden coverage limits are the second standard restriction, usually expressed as a maximum proportion of the garden or of the unbuilt curtilage that may be excavated. The purpose is partly visual and partly hydrological, because a continuous concrete slab under a garden changes how rainfall reaches the ground.

Alongside the coverage limit, most policies set a minimum depth of soil to be retained over any part of the basement beneath a garden, so that planting can establish and trees can survive. That requirement interacts badly with waterproofing and drainage layers, because the soil depth is measured above the finished build-up rather than above the structural slab. Designing to the minimum leaves no tolerance.

Rainwater that can no longer soak away has to go somewhere. Expect to need a Drainage Strategy showing how surface water is managed across the whole plot after construction, including attenuation where the policy requires no increase in run-off. On a small urban plot, that often means a tank and a flow control device in the only part of the garden you were not excavating.

Light wells and front gardens

Front light wells are the detail that attracts the most objections and the most refusals. They are visible from the street, they interrupt the rhythm of a terrace, and they usually need guarding or railings that read as an addition.

Policies commonly restrict front light wells to the width of an existing opening, require them to sit behind the building line, limit their projection into the front garden, or rule them out in a conservation area altogether. Rear light wells are treated far more leniently, and in a scheme where natural light is needed, the design should put the habitable rooms at the rear.

Where a light well is the only way to provide natural light and escape, the fire strategy and the planning design are the same conversation. Ventilation, guarding and the safe egress arrangements for habitable basement rooms should be resolved on the same drawing as the planning elevation.

Heritage and conservation area constraints

A basement under a listed building needs listed building consent as well as planning permission, and the two are assessed on different tests. The consent question is whether the works affect the character of the building as one of special architectural or historic interest, and excavation under a historic structure almost always does, because it affects the foundations, the floors and often the plan form of the lower ground floor.

In conservation areas the issues are the light well, the construction impact on the street scene and the loss of garden space, which in many areas contributes to the character of the designated area. Boroughs with large conservation area coverage tend to have the most restrictive basement policies, which is not a coincidence.

Archaeology matters more than people expect in central London, where basement excavation is often the first ground disturbance below made ground on a site. Where the site sits in an archaeological priority area, expect a condition requiring evaluation or a watching brief.

The impact assessment the policy asks for

Where a policy requires a basement impact assessment, it usually requires it in a defined sequence: screening to identify which issues are relevant, scoping to decide what investigation is needed, site investigation and study, then impact assessment with mitigation. A Basement Impact Assessment is a ground engineering document, and councils expect a suitably qualified author. Professional review is strongly advised, because the structural and hydrogeological conclusions carry liability.

The investigation normally covers ground conditions and stratigraphy, groundwater levels including any perched water, the stability of adjoining structures, surface water flows, trees and their root protection areas, and the construction methodology and sequence. Where the site sits on river terrace gravels over London Clay, groundwater is likely to be the controlling issue. Where it sits on clay alone, the controlling issue is usually heave and the effect on neighbouring foundations.

A Geotechnical Assessment to Eurocode 7 and a waterproofing design to BS 8102 sit behind the planning submission. So does a construction methodology statement, and most boroughs require a Construction Management Plan by condition covering lorry routes, hours, wheel washing and neighbour liaison.

Groundwater monitoring is the item that most often delays an application, because a single dip reading tells you nothing about seasonal variation and authorities increasingly ask for monitoring over several months. Start it early.

Reading the adopted policy before you design

Three practical steps before the first sketch. Download the adopted local plan and search it for the word basement, then do the same for any supplementary planning document or design guide. Check whether the council has published a validation checklist entry for basement applications, because that tells you exactly which documents will be required. Then check the policy's own definitions, because terms like garden, curtilage and storey are often defined locally in a way that changes the answer.

One more point that catches householders. Basement floorspace counts towards gross internal area for Community Infrastructure Levy purposes. The minor development exemption in the CIL Regulations applies to new build of less than 100m² gross internal area that does not create a dwelling, so a large basement can tip an otherwise exempt extension into a chargeable scheme. Check the council's charging schedule rate and the exemption position before you commit, and claim any self-build or residential extension exemption before works start.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

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