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City of London, London

Basement impact assessment in the City of London

In the City of London a basement means something different from a London suburb: only 2 per cent of households live in a house, gardens cover just 0.1 per cent of the land and 637 entries are listed. Here the assessment concerns a building and its neighbours. We coordinate it on a fixed quote after a free constraints check.

Fixed quote after a free constraints check

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    Architectural model of a four-storey London stucco townhouse in its terrace, with a portico, area railings, sash windows, a cornice and chimney stacks
    The house as it stands: four storeys over a lower ground floor.Existing Proposed worksAn illustrative model, built for this page.

    How we work on basement impact assessments

    Seven steps from first call to approval.

    The same model cut through the middle: every floor, the stair, the chimney breasts, the new mansard, rear extension and basement in section
    Section through the model: every floor on one datum.

    1. Enquiry and constraints check

      We check the property against conservation areas, listed status, Article 4 directions, tree preservation orders, flood zone and planning history. We choose the right route and send a fixed quote.

    2. Measured survey

      We survey the house on one datum: rooms, levels, boundaries, drains, trees and the neighbours' windows that the 45° and 25° tests depend on.

    3. Design options

      We model the house in Revit and show you options in 3D and plan, with the planning strategy and the precedents in your street.

    4. Design frozen

      You choose an option in writing. Nothing changes after this without your say.

    5. Independent check

      A second chartered technologist checks every sheet against the council's validation checklist and the policy. A senior review signs it off before anything is submitted.

    6. Submission

      We prepare the forms, certificates, fee and documents and submit through the Planning Portal.

    7. Officer liaison

      We answer the planning officer and make any amendments within five working days, at no charge until approval.

    Free redraws until approval. Every drawing set is checked independently, and the checker is never the person who drew it.

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    Rear view of brick terraced houses in London with slate roofs and chimney stacks along the shared roofline

    What a basement impact assessment means in the City of London

    The City of London Corporation is the planning authority for the Square Mile, and its development plan is the City of London Local Plan 2015. We have not found a published residential basement guide with figures that we can quote, so this page does not state a numeric limit.

    What we do is read the plan and the supplementary guidance against your building. In the Square Mile that usually means asking different questions from the suburbs: who owns the structure, whether the floor you want is in a flat with a shared frame, and what lies beneath and beside the building.

    The report itself still follows the same logic as anywhere in London: ground stability, groundwater and surface water. The difference is that the engineer is often looking at an existing deep basement or a vault, not at soil under a lawn.

    Pencil resting on a dimensioned technical drawing beside a scale rule

    Local figures for City of London

    Figures with their unit, period and source
    MeasureValuePeriodSource
    Conservation areas28 named conservation areas published by the council—planning.data.gov.uk
    Article 4 directions1 Article 4 directions published by the council—planning.data.gov.uk
    Listed building entries637 list entries (Historic England, within the LPA boundary)—planning.data.gov.uk
    Heritage at Risk entries5 Heritage at Risk Register entries within the LPA boundary (Historic England)—planning.data.gov.uk
    Households living in a house2 % of households living in a house or bungalow (detached, semi-detached or terraced)Census 2021nomisweb.co.uk
    Owner-occupied households36.5 % of households owner-occupied (owned outright or with mortgage/loan)Census 2021nomisweb.co.uk
    Domestic gardens share of land (GLUD 2005)0.1 % of land that is domestic garden (GLUD 2005)2005data.london.gov.uk
    Local planCity of London Local Plan 2015—cityoflondon.gov.uk
    Borough CIL residential rate£251.05/m² (Riverside); £159.00/m² (Rest of City) [schedule rates £150 and £95]Indexed rate for planning permissions granted in 2026 (Annual CIL Rate Summary 2026, effective 1 January to 31 December 2026)cityoflondon.gov.uk
    Party Wall etc. Act 1996 s6(1): excavation within 3 m horizontally of an adjoining owner's building or structure and deeper than the bottom of its foundations3 m—legislation.gov.uk

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

    Councils bordering City of London
    Which application your project needsLawful Development Certificate decision: 8 weeks. Prior approval decision: 42 days. Householder decision: 8 weeksYour projectWithin permitted development?Lawful Development CertificatePrior approvalHouseholder applicationYesLarger rear extensionNoABC

    Which application your project needs

    The usual route for a house; flats and listed buildings follow other rules.

    A · Lawful Development Certificate decision
    8 weeks
    B · Prior approval decision
    42 days
    C · Householder decision
    8 weeks

    Heritage and housing figures that shape a City basement

    The City has 28 conservation areas and 1 Article 4 direction, so a conservation area is the default setting, not the exception. Historic England lists 637 entries, of which 445 are Grade II, and 5 appear on the Heritage at Risk Register.

    The best-known residential example is the Barbican estate, covered by the Barbican Estate Listed Building Management Guidelines: Volume I Introduction and Volume II Residential (SPD). If your flat is in the estate, the guidelines decide what can change inside and outside the listed building, and a basement-level alteration needs to be read against them.

    Only 36.5 per cent of households are owner-occupied, so consent from a freeholder or managing agent is usually the first step, long before a planning application.

    Why the householder statistics say so little

    The City decided 1 householder application in its latest year, and granted 100 per cent of it. A single decision tells you nothing about how the council would treat a basement, so we do not rely on it.

    Instead we look at the building. A flat or a commercial unit is judged on heritage, access and construction impact rather than on garden ratios, because there is rarely a garden to measure.

    That is why the free constraints check matters more in the City than anywhere else. It tells us whether your project is a householder scheme, a change to a flat or something larger, and which documents belong with it. It also tells you early if the work needs the freeholder's licence, which can take longer than the planning decision itself.

    We then agree one set of sheets for every party: the existing structure surveyed and drawn, the proposed section showing the new floor level against adjoining foundations and any services, and a construction note on how materials and spoil will be handled on a site with little room for either.

    Levy, party wall and Building Regulations in the Square Mile

    The City's Community Infrastructure Levy for residential floorspace is £251.05/m² (Riverside); £159.00/m² (Rest of City) [schedule rates £150 and £95], and the national rules give no basement-specific exemption. The mayoral levy adds 96.97 pounds per square metre once indexed.

    Tight sites make party wall duties unavoidable. Excavation within 3 metres of an adjoining building that goes deeper than its foundations needs a notice, as does a dig within 6 metres on the diagonal test, and work to an existing party structure needs 2 months' notice.

    Building Regulations require drainage or protection where the water table can rise within 0.25 metres of the lowest floor, and the engineer's design covers the structure as well as the water.

    Price and next steps

    The service has no list price. After the free check you receive a fixed quote, and we say plainly if your project does not need a report. Our planning drawings in the City of London cover the application that goes with it.

    For the borough overview, see architectural drawings in the City of London. Just to the west, Westminster publishes numeric basement limits, and the national rules are on the basement impact assessment hub.

    What you receive

    Planning drawing sheet for a rear extension with plans, elevations and sections
    Sample drawing. Planning drawings for a rear extension: plans, elevations and sections on one sheet set.
    Rows of brick terraced houses and gardens on a residential street
    The street around you
    Move across the plinth — the blocks rise around the pointer.

    Other services in City of London

    Basement impact assessments nearby

    Questions

    Does the City of London need a basement impact assessment?

    It depends on the building and the works. We read the local plan against your property and tell you in writing whether a report is likely to be asked for.

    Can I dig a basement in a Barbican flat?

    The Barbican estate is covered by listed building management guidelines, so any change has to be read against them first. We start with the guidelines and the freeholder's consent.

    Do I need a party wall notice in the City?

    Usually, if the dig is within 3 metres of a neighbouring building and deeper than its foundations, or within 6 metres on the diagonal test.

    What does it cost?

    A fixed quote after a free constraints check.

    Sources and dates

    City of London skyline glowing at dusk across the Thames under a dramatic orange sky

    Tell us about your property.

    We check the constraints, then send a fixed fee. Messages before 3pm get a reply the same working day.

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