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Bright finished basement room with a glass-balustrade staircase, high windows and a built-in desk
Photograph: Point3D Commercial Imaging Ltd.

01Mansards, extensions and basements

Basement conversion drawings for London's period houses

A basement under a period house is a planning, structural and neighbour question at once, and prime London councils set firm limits on how much garden it may take and how deep it may go. We draw to those limits from £1,995 including VAT, and coordinate the structural calculations that sit behind them.

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    02The brief

    Four councils, four ways of limiting a basement

    Westminster supports a basement under no more than 50% of the garden land and no more than 1 storey below the lowest original floor. Kensington and Chelsea sets the same ceiling on extent, 50% of each garden or open part of the site, and existing lightwells count towards that 50%.

    Camden reads a single storey as roughly 3-4 m of excavation. Haringey frames its test around the front: it asks whether a lightwell would take more than 50% of the front garden or amenity area. The same house plan can pass in one borough and fail in the next.

    Georgian brick terrace with sash windows, arched fanlight doorways, stone steps and iron railings in London
    Rear view of brick terraced houses in London with slate roofs and chimney stacks along the shared roofline
    Photograph: Ben Hadfield

    03The record

    The national rules in figures

    Figures with their unit, period and source
    MeasureValuePeriodSource
    Basements should not extend beneath more than 50% of the garden land; where the garden is 8m or less at its longest point they may extend up to 4m from the original building.50%No periodwestminster.gov.uk, City Plan 2026, Policy 50, p.203, Policy 50 B1
    Basements should not comprise more than one storey beneath the lowest original floor level, except on large sites with high accessibility for construction.1 storeyNo periodwestminster.gov.uk, City Plan 2026, Policy 50, p.203, Policy 50 B3
    Basements must not exceed 50 per cent of each garden or open part of the site, with the unaffected garden kept in a single area; exceptions may be made on large sites.50%No periodrbkc.gov.uk, Local Plan 2024, Policy CD11, p.251, CD11 A
    Basements must not comprise more than one storey (exceptions on large sites) and no further basement floors may be added where a basement already has permission or was built.1 storeyNo periodrbkc.gov.uk, Local Plan 2024, Policy CD11, pp.251-252, CD11 B-C
    Basements must include a sustainable drainage system and a minimum of one metre of soil above any part of the basement beneath a garden.1 m soilNo periodrbkc.gov.uk, Local Plan 2024, Policy CD11, p.252, CD11 I-J
    A single basement storey is taken as approximately 3 to 4 metres of total excavation depth (external dimensions).3-4 mNo periodcamden.gov.uk, Basements CPG, p.11, Policy A5 criterion f
    A minimum of 1 metre of soil is expected above basement development extending beyond the building footprint.1 m soilNo periodcamden.gov.uk, Basements CPG, p.20, para 3.2
    Light wells are assessed on whether they would lose more than 50% of the front garden or amenity area (Policy DM18 C).50%No periodharingey.gov.uk, Development Management DPD 2017, p.38, Policy DM18 C(c)
    Party Wall etc. Act 1996 s6(1), s6(2), s6(5)Adjacent excavation notice at least 1 month before work: within 3 m and deeper than the neighbour's foundations, or within 6 m and cutting a 45° line from their foundationsNo periodlegislation.gov.uk
    Party Wall etc. Act 1996 s5, s6(7)A dispute arises if the adjoining owner does not consent within 14 days of the noticeNo periodlegislation.gov.uk
    Approved Document B Vol 1, para 2.16Basement storeys with habitable rooms need an escape window or external door, or a protected stair to a final exitNo periodgov.uk
    Householder: alterations/extensions to a single dwellinghouse, from 1 Apr 2026548 GBPNo periodribblevalley.gov.uk

    A blank value means the figure has not been published, not that there are none. Check your own address with the council.

    London33 councils
    Councils on this page

    04The drawing

    A basement in sectionA, Party wall notice for excavation: Adjacent excavation notice at least 1 month before work: within 3 m and deeper than the neighbour's foundations, or within 6 m and cutting a 45° line from their foundations. B, Escape from basement rooms: Basement storeys with habitable rooms need an escape window or external door, or a protected stair to a final exitAB

    A basement in section

    A house and garden over a single-storey basement. The lettered marks show the limits the legend gives.

    A Party wall notice for excavation
    Adjacent excavation notice at least 1 month before work: within 3 m and deeper than the neighbour's foundations, or within 6 m and cutting a 45° line from their foundations
    B Escape from basement rooms
    Basement storeys with habitable rooms need an escape window or external door, or a protected stair to a final exit
    The basement in three dimensions3D
    Scroll the page: the model turns with it.

    05The detail

    Soil depth and the garden above

    A basement that extends beyond the footprint of the house leaves a garden on its roof, and councils write rules for that garden. The Royal Borough of Kensington and Chelsea requires 1 m soil over any part of a basement beneath a garden, and Camden expects 1 m soil above development beyond the building footprint.

    These depths decide how deep the slab sits and what the structural engineer designs, so we fix them at the first design option and not after it.

    Article 4 and the period front garden

    In Kensington and Chelsea a direction across the whole borough takes away the permitted development right to extend a house downwards: An Article 4 direction covering the whole Royal Borough of Kensington and Chelsea removes permitted development rights for basement extensions to single dwelling houses; it came into force on 28 April 2016. Planning permission is therefore the starting point for any basement there, listed or not.

    The appraisals show what officers dislike. The Holland Park appraisal records the harm plainly. Excavated front gardens to light basements have removed greenery and left a gaping hole in front of houses. Our Holland Park guide sets out how we draw a lightwell that avoids that harm.

    Party walls, escape and fire detection

    Digging beside a neighbour brings the Party Wall etc. Act 1996 into play. Adjacent excavation notice at least 1 month before work: within 3 m and deeper than the neighbour's foundations, or within 6 m and cutting a 45° line from their foundations. A dispute arises if the adjoining owner does not consent within 14 days of the notice.

    Building Regulations add their own demands. Basement storeys with habitable rooms need an escape window or external door, or a protected stair to a final exit. All dwellings: fire detection and alarm system minimum Grade D2 Category LD3 (BS 5839-6), mains-operated smoke alarms to BS EN 14604. Our drawings carry both from the first issue, and the council's householder fee in the national table is £548, paid to the council separately.

    Where basement work meets listing

    A listed house needs consent for the basement as well as permission for it, and the two applications must tell the same story. Read the route under listed building consent drawings before you commission a survey.

    For a house on the Kensington and Chelsea border, Knightsbridge shows how two councils' basement rules can apply to one neighbourhood.

    Planning sheet for a single-storey rear extension: proposed ground floor plan and rear elevation, with the Property Architecture title block
    Sample drawing. Proposed plan and rear elevation from a planning application set, sheet 03 at 1:50.

    06The papers

    What the drawings rest on

    Brick house being extended, with a wide new opening supported over the ground floorStructural calculations
    Photograph: Brett Jordan
    Structural calculation sheet for a steel beam over a new rear opening, with the load diagram, loading table and section checks
    Sample drawing. Beam B1 over a new rear opening: loading, bending moment and section checks.

    07Correspondence

    Questions

    How much of my garden can a basement take?

    It depends on the council. Westminster and Kensington and Chelsea both limit a basement to 50% of the garden, while Camden adds a size test against the house itself (1.5 x footprint). We read the rule for your address before drawing.

    How deep can a London basement go?

    Westminster and Kensington and Chelsea allow 1 storey, with exceptions only on large sites. Camden treats one storey as roughly 3-4 m of excavation.

    What do the drawings cost?

    Basement drawings start at £1,995 including VAT. Council fees are separate, and structural calculations and a basement impact assessment are coordinated alongside where the council asks for them.

    Do party wall rules change from one council to the next?

    They do not. The Act is national, so the notice period and the distances are the same in every borough: Adjacent excavation notice at least 1 month before work: within 3 m and deeper than the neighbour's foundations, or within 6 m and cutting a 45° line from their foundations. What differs from house to house is the number of neighbours. A terrace has two, each owner gets a notice, and the drawing for each starts at £45.

    Rows of brick terraced houses and gardens on a residential street
    Photograph: Altaf Shah

    08Further reading

    London

    09Sources

    Sources and dates

    Licences

    Kensington street lined with brick townhouses and lit streetlamps beneath an orange dusk sky

    Send us the address of your house.

    We look up the conservation area, listing and Article 4 position, then reply with a fixed fee. A message before 3pm is answered the same working day.

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    Basement conversion drawings for London period houses