Basements and Subterranean Space

Basement policy in Kensington and Chelsea sits at Policy CD11 of the Local Plan adopted in July 2024. It replaced Policy CL7, and any adviser still citing CL7 is working from a superseded plan. The substance carried across largely unchanged, but the reference has moved.

The two constraints that determine whether a scheme is possible at all are simple to state. A basement must not exceed fifty per cent of each garden or open part of the site, with the unaffected garden left in a single area that where relevant forms a continuous run with neighbouring gardens. And it must not comprise more than one storey. Exceptions to both may be made on large sites, which in practice means very few houses.

A single storey means one that cannot be subdivided later to create additional floors, generally taken as around three to four metres floor to ceiling, with a small additional allowance sometimes permitted for a swimming pool. Extent is measured as gross external area. Where a basement already exists under an extant or implemented permission, or was built under permitted development before those rights were withdrawn, no further basement floor may be added beneath it.

Beyond the envelope, the policy requires a minimum of one metre of soil above any part of the basement lying beneath a garden, a sustainable drainage system retained thereafter, and protection from sewer flooding by a suitable pumped device. Trees of townscape or amenity value must not be lost, damaged or placed under long-term threat, which on these plots is frequently the constraint that reshapes the footprint.

CD11 forbids excavation underneath a listed building, and the prohibition expressly includes vaults. There is no balancing exercise offered and no exception for large sites.

The reasoning connects back to what a listing protects. RBKC treats the historic floor plans and levels of a listed building, and its structure including its foundations, as part of its special interest. Inserting a new floor below the original lowest level of the house damages the hierarchy of historic floor levels, whatever the finish of the new room.

For a listed house, then, the question is not how large a basement can be but whether any part of the scheme can go under the garden instead, clear of the building footprint, within the fifty per cent limit and under a metre of soil. That is often achievable and it is a different project from the one the owner arrived with. Establishing which of the two is on the table is the first thing to settle, because it changes the budget by a seven-figure margin.

A basement is invisible until it needs daylight, ventilation, plant and a means of escape, and each of those wants to surface somewhere.

CD11 resists light wells and railings to the front or side where they would seriously harm the character and appearance of the locality, and particularly where they are not already an established and positive feature of the streetscape. On terraces where a run of original front light wells and railings exists, a new one that matches can be argued. On a street where they do not, it will not be.

External elements including light wells, roof lights, plant and escape routes must be sensitively designed and discreetly sited, and light pollution has to be limited. In practice the escape strategy and the plant location are worked out at the same time as the structural scheme rather than afterwards, because a scheme that resolves the structure and then discovers it needs a ventilation grille in the front garden has to start again.

RBKC does not use the term Basement Impact Assessment, which belongs to Camden. The document required here is a Construction Method Statement, prepared by an appropriately qualified civil or structural engineer. It accompanies the planning application without forming part of it.

It addresses the ground and hydrological conditions of the site, including groundwater flow, and demonstrates how the excavation, the demolition and the construction, including the temporary propping through every stage, can be carried out safely.

The Local Plan is unusually candid about the limits of its own jurisdiction here, and the point is worth understanding. The structural stability of the development itself is not controlled through the planning system but through building regulations, and the Council states that the Party Wall Act is better suited to dealing with damage-related issues. Planning controls the size, the appearance and the disturbance. Whether the house stands up is a matter for the engineer, building control and the party wall surveyors.

Construction Method Statement

The engineer's account of ground conditions, groundwater and the temporary works sequence. Accompanies the application without forming part of it.

Draft CTMP

A construction traffic management plan submitted with the application. Where permission is granted, a condition requires the full version before work starts.

Section 61 Consent

Prior consent for noise under the Control of Pollution Act 1974, required on Category 1 sites. Basement excavation is always Category 1.

Construction Bond

Lodged with the Council before works begin on Category 1 and 2 sites, and refunded less the Council's monitoring costs at the end.

A basement under a Holland Park terrace almost always engages the Party Wall etc. Act on more than one front at once, because underpinning a shared wall is work to a party structure under section 2 and the excavation itself is notifiable under section 6.

Section 2 requires two months' notice. Section 6 requires one month, and applies where the dig goes within three metres of a neighbour's structure and below the level of their foundations, or within six metres where it meets a line drawn downwards at forty-five degrees from the base of those foundations. On a standard terrace both flanks and often the rear will be caught.

Where an adjoining owner does not respond within fourteen days, a dispute is deemed to have arisen and surveyors are appointed. The building owner ordinarily bears the cost of the award and of the adjoining owner's surveyor. RBKC's Local Plan recommends beginning these negotiations before the planning application goes in, and on a basement that should be treated as the default rather than as advice.

The logistics are a real constraint here rather than a paragraph of reassurance, and they are governed by the borough's Code of Construction Practice. Basement excavation is characterised as a Category 1 site, the highest impact classification, which brings the full weight of that code with it.

Noisy works are permitted from eight in the morning to six in the evening on weekdays, and at no time on Saturdays, Sundays or bank holidays. RBKC is stricter than most London boroughs on this, several of which allow Saturday morning working. High impact activities, which include all demolition and concrete breaking, are confined to two windows on weekdays: nine until noon, and two until half past five. Deliveries and collections run from half past nine to half past four. Percussive breaking equipment is to be avoided wherever it can be.

A construction bond is lodged before works start, currently eight and a half thousand pounds including VAT for each year or part year of the projected programme on a Category 1 domestic site, refunded less the Council's monitoring costs. Every effort must be made to locate the site compound and the skip within the site itself rather than on the highway. Vehicles must meet ULEZ requirements and must not idle.

None of this is unworkable, but it dictates the programme rather than decorating it. A dig priced on eight-hour days and Saturday working is a dig priced wrongly. The Council also requires traffic plans to account for other active or permitted construction nearby, which on these streets is a live consideration: basement applications in the borough rose from forty-six in 2001 to four hundred and fifty in 2013, and multiple simultaneous excavations in a single street are not unusual.