Basement Conversion in London: Planning, Building Regulations and the Rules That Decide the Design

Basement conversion in London is governed from two directions at once, and people usually discover the second one late. Planning decides how big the box can be, and in Camden that means one storey and no more than half of each garden. Building Regulations decide whether it is legally habitable once it exists, and the two tests that catch most schemes are means of escape and waterproofing. Neither is negotiable, and both are cheaper to design in than to retrofit.

This guide covers when you need planning permission, the borough rules that set the size, the parts of the Building Regulations that actually apply, the waterproofing standard that replaced the withdrawn guidance, and the Party Wall notices that set your programme.

Two different projects, one word

Before anything else, work out which job you are doing, because the cost, the consents and the risk are not comparable.

• Converting an existing cellar. The void is already there and has usable height. This is a fit-out with waterproofing, ventilation, lighting and escape added. No excavation, no underpinning, usually no planning application.
• Excavating a new basement, or digging out an existing one. This is a structural engineering project. Underpinning, temporary works, a structural engineer, a party wall process, and in several boroughs a Basement Impact Assessment. It is the version that appears in the newspapers.

Everything below applies to both. The planning section applies mainly to the second.

Planning permission: what triggers it

Converting a cellar into a habitable room, with no change to the external appearance of the building and no change of use, generally does not require planning permission. What triggers an application is anything that changes the outside or the ground: a new lightwell, external steps, a separate entrance, a raised pavement vault, or excavation beyond the existing footprint.

Two overrides to keep in mind. In a conservation area the permitted development rights that would otherwise cover small external changes are narrowed, and on a listed building any internal alteration can need listed building consent regardless of how invisible it is. Our guides to renovating in a conservation area and listed building interior alterations set out the difference, and planning permission for internal alterations covers the general rule.

The borough rules that set the size

This is where London differs from everywhere else. Several boroughs adopted specific basement policies after a decade of large excavations, and those policies, not the Building Regulations, decide the shape of the scheme.

Camden, Local Plan Policy A5

Camden's is the most prescriptive and the most useful to read even if you are building elsewhere, because other boroughs borrowed from it. A basement development should:

• not comprise more than one storey. Camden treats a single storey as approximately 3 to 4 metres in height, and that figure refers to the total depth of the excavation measured externally, not the finished ceiling height. A deeper section may be allowed for a swimming pool, but only for a swimming pool.
• not be built under an existing basement.
• not exceed 50 per cent of each garden within the property. This is applied to the front, rear and side gardens individually, not as a total across the property, and to the gardens as they exist now rather than as proposed. The unaffected garden has to be a single area, and where it matters should join up with neighbouring gardens.
• be less than 1.5 times the footprint of the host building in area.
• extend into the garden no further than 50 per cent of the depth of the host building, measured from the principal rear elevation.
• not extend into or underneath the garden further than 50 per cent of the depth of the garden.
• be set back from neighbouring boundaries where it goes beyond the footprint of the host building.

These criteria are read together, so meeting one does not excuse failing another. Camden also requires a site-specific Basement Impact Assessment at application stage, running through screening, scoping, site investigation, impact assessment and review, prepared by engineering professionals holding qualifications the council specifies, and most are then independently audited.

One definition worth knowing: a storey that is partly below ground is not automatically a basement. To count as a lower ground floor rather than a basement, Camden expects it to have a significant proportion above the prevailing ground level, to be accessible from outside at front and rear, to be part of the original fabric, and to be part of the character of the area.

Kensington and Chelsea

The Royal Borough's approach is the other well-known one: one storey only, and no basement covering more than 50 per cent of each garden or open part of the site. The detail sits in the Basements Supplementary Planning Document adopted in April 2016, which was written to support Local Plan Policy CL7 on basements.

Everywhere else

Do not assume. Westminster, Hammersmith and Fulham, Islington and others all have their own basement policies, and several inner boroughs added them at different dates. Read the current Local Plan policy for your borough before the architect draws anything, because the size limits are the design brief.

Building Regulations: the parts that apply

There is no part of the Building Regulations called basements. The requirements arrive from six directions.

Part A, structure

Underpinning, retaining structures and temporary works all need design by a structural engineer, with calculations submitted to building control. A full plans application is the sensible route for a basement rather than a building notice, because you want the structural design approved before anyone starts digging. Our guide to removing a load-bearing wall covers the same submission process on a smaller scale.

Part B, fire safety

This is the one that most often forces a design change. A basement storey containing a habitable room needs either an emergency escape window or external door giving escape from the basement, or a protected stairway leading from the basement to a final exit.

If you are relying on a window, Approved Document B sets the dimensions: an unobstructed openable area of at least 0.33 square metres, at least 450mm high and at least 450mm wide, with the bottom of the openable area no more than 1,100mm above the floor. A window can never be the final exit itself, so it has to open into a lightwell or area from which a person can get to ground level.

If you are relying on a protected stairway instead, the stair enclosure has to be fire-resisting with fire doors, which usually means giving up an open-plan connection between the basement and the ground floor. That is the design decision to make early, not late. Our guides to escape window regulations and where fire doors are required go into the detail.

Part C, moisture, and BS 8102

Part C requires the floors, walls and roof of a building to resist the passage of moisture to the inside. What it no longer does is tell you how. Basements for dwellings was withdrawn from the Approved Documents on 1 October 2010, and no Approved Document has covered below-ground waterproofing design since.

The standard that fills the gap is BS 8102, the code of practice for protection of below ground structures against water ingress, revised in 2022. It sets out three types of protection and expects the design to be led by someone competent in waterproofing:

• Type A, barrier protection: a tanking membrane or cementitious slurry applied to the structure.
• Type B, structurally integral protection: the concrete structure itself is designed to be water resistant.
• Type C, drained protection: a cavity drain membrane behind the finishes, collecting water to a sump and pump.

For a habitable room you are aiming at Grade 3, a dry environment. In most London conversions that means a combined approach rather than one type alone, and it means a maintainable system: a cavity drain membrane with an inaccessible pump under a finished floor is a future flood, not a specification. Building control will ask who designed the waterproofing and against which grade.

Part K, stairs

Headroom over the stair has to be at least 2 metres measured along the pitch line, including over landings. The relaxation that allows 1.9 metres in the middle and 1.8 metres at one side applies to loft conversions, not to basements, so plan the stair opening around the full 2 metres. Our staircase building regulations guide has the going, rise and handrail figures.

Parts F, L, G and P

Ventilation under Part F matters more below ground than above it, because you have no passive stack and often no openable window on two sides. Part L applies to the new thermal elements you create. Part G applies if you are adding a bathroom or a kitchen. Part P covers notifiable electrical work, which a basement conversion always involves. Our guides to extractor fan regulations, Part P and building regulations for a London renovation cover each.

One thing that does not apply: there is no minimum ceiling height for a habitable room in the Building Regulations in England. The constraint is the 2 metres over the stair, not a number for the room. Our guide to minimum ceiling heights explains why that surprises people.

The Party Wall Act sets your programme

Under section 6 of the Party Wall etc. Act 1996 you must serve notice on an adjoining owner if you intend to excavate within 3 metres of their building or structure and to a depth below the level of their foundations, or within 6 metres where a line drawn downwards at 45 degrees from the bottom of their foundation would intersect your excavation. At least one month's notice is required before work starts.

In practice the month is the floor, not the timetable. If the neighbour dissents, surveyors are appointed, a schedule of condition is prepared and an award is drawn up, and on a basement that routinely takes several months. Serve early. Our Party Wall Act guide for London renovations covers the notices and the costs.

A sensible order of work

1. Read your borough's current basement policy and size the scheme to it.
2. Appoint a structural engineer and, separately, a waterproofing designer competent under BS 8102.
3. Decide the escape strategy, window or protected stair, before the layout is fixed.
4. Submit a full plans building control application with the structural calculations.
5. Serve party wall notices, allowing months rather than weeks.
6. Where required, commission the Basement Impact Assessment with the planning application, not after it.

If the answer to all of that is that the basement is not worth it, a loft is often the better return on the same money: see our loft conversion building regulations guide. For the rooms themselves, our styles guide and the Vertigo Interiors home page cover the design side.

Frequently asked questions

Do I need planning permission for a basement conversion in London?

Converting an existing cellar into habitable space, with no change to the outside of the building and no change of use, usually does not need planning permission. Excavating a new basement, or adding a lightwell, external steps or a separate entrance, usually does. In a conservation area or a listed building, assume you need consent and check before you design anything.

How deep can a London basement go?

In Camden, one storey, which the council treats as roughly 3 to 4 metres of total excavation depth measured externally. It also has to sit no more than 1.5 times the footprint of the host building in area, extend into the garden no further than half the depth of the host building measured from the rear elevation, cover no more than half of each garden, and not be built under an existing basement. Other boroughs set their own limits, so check yours.

What does Building Control need for a basement?

Structural calculations under Part A for the underpinning and the temporary works, a means of escape under Part B, waterproofing designed to BS 8102, ventilation under Part F, insulation under Part L, notifiable electrical work under Part P, and stairs that meet Part K including 2 metres of headroom along the pitch line. A full plans application is normal for this work rather than a building notice.

Does a basement bedroom need a window?

It needs a means of escape, which is not quite the same thing. A basement storey with a habitable room needs either an emergency escape window or external door giving escape from the basement, or a protected stairway leading to a final exit. An escape window needs an unobstructed openable area of at least 0.33 square metres, at least 450mm in each direction, with the bottom of the opening no more than 1,100mm above the floor. A window is never acceptable as the final exit itself.

Is tanking covered by Building Regulations?

Not by an Approved Document. Basements for dwellings was removed from the Approved Documents on 1 October 2010, so there is no official guidance document for below-ground waterproofing design. The requirement in Part C still applies, and BS 8102:2022 is the code of practice building control will expect you to have followed. Grade 3 internal environment is the standard for habitable rooms.

Do I need a Party Wall notice?

Almost certainly. Under section 6 of the Party Wall etc. Act 1996 you must notify an adjoining owner if you are excavating within 3 metres of their structure and to a depth below their foundations, or within 6 metres where a line drawn down at 45 degrees from their foundation would meet your excavation. At least one month's notice is required, and for a basement the surveyors' process usually takes longer than that.

How much does a London basement conversion cost?

It splits into two very different jobs. Converting an existing cellar that already has usable height is a fit-out with waterproofing attached. Digging out and underpinning to create new height is a structural project with temporary works, engineers, party wall surveyors and a Basement Impact Assessment, and it costs several times more. Establish which one you are doing before anyone quotes.

Sources

• Camden Planning Guidance: Basements, for the Policy A5 criteria on storeys, garden coverage and footprint, and the Basement Impact Assessment process
• Approved Document B, Volume 1: Dwellings, for the basement escape provisions and the escape window dimensions
• Royal Borough of Kensington and Chelsea: Basements SPD, for the one storey and 50 per cent garden limits

Checked on 14 September 2026. Guidance applies to England, and borough basement policies are reviewed periodically, so confirm the current Local Plan policy for your address. Building control can accept an alternative approach where it meets the requirement, so agree any departure before work starts.