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Planning

Basement impact assessment in London

A basement impact assessment is only needed where your borough's policy asks for it, and many London boroughs do once you dig beside neighbours, groundwater or trees. It is a technical report that sits beside your planning drawings, not a drawing set in its own right. We coordinate it on a fixed quote after a free constraints check, which also tells you whether your council will want one.

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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    What a basement impact assessment shows the council

    A new floor below an old house changes three things at once: how water moves through the ground, how the soil carries the building's weight, and how the neighbouring walls behave while you dig. The assessment is the evidence that none of those changes will harm your home, the adjoining houses or the street.

    No national rule demands it. The London Plan leaves large basements to individual boroughs, so the duty to submit a report comes from local policy, and the trigger depth, the level of detail and the people who must sign it off all differ between councils.

    That is why we start with your address and the depth you have in mind, then read the borough's own policy before a line of design is drawn. Everything else follows from what that policy says.

    Rows of brick terraced houses and gardens on a residential street

    Screening, scoping, investigation and assessment

    The most widely copied method is Camden's, which moves through four steps: screening, scoping, site investigation and the impact assessment. Screening is a set of short questions about slope stability, groundwater and surface water. Each question answered with a yes becomes an item for scoping, where the specialist work is defined.

    The three themes that run through the report are ground stability, groundwater and surface water. Stability looks at the soil and the neighbours' foundations. Groundwater asks whether the new box will block a natural flow. Surface water covers drainage and flood risk.

    Camden's approach rests on a published geological and hydrological study by Arup. Kensington and Chelsea takes a different line with its own detailed guidance and firm numerical limits, which is the better illustration of how far boroughs can differ.

    Numbers that can fix the size of a basement

    Kensington and Chelsea's guidance has limited a basement to no more than 50 per cent of each garden, and asks for at least 1 metre of soil over any part of a basement that lies beneath a garden. The borough has since revised its local plan, so we confirm the current wording before relying on it. Other councils set other figures, and none of these is a London-wide rule.

    Building Regulations then add their own checks. If the water table can rise to within 0.25 metres of the lowest floor, the floor has to be drained by gravity or another effective means. Part A separately requires that ground movement must not impair the stability of the building.

    A design that breaks a local limit has to be redrawn, so we match the sections and plans to the council's figures first.

    Digging down triggers party wall duties

    The Party Wall etc. Act 1996 is separate from planning and applies to excavation close to a neighbour. A notice is needed where you dig within 3 metres of an adjoining building and go deeper than its foundations, and within 6 metres where a 45-degree line drawn from the bottom of its foundations meets your excavation.

    An excavation notice must be served at least 1 month before work starts, with plans and sections showing the site and the depth. Work to an existing party structure needs 2 months. If a neighbour does not reply in writing within 14 days, a dispute is treated as having arisen, and surveyors take over.

    Our party wall notices service prepares the drawings that accompany the notice. If a neighbour asks for their foundations to be underpinned, that cost falls on the building owner.

    A basement is a team job: who does what

    A structural engineer designs the retaining walls and the slab. A ground investigation firm drills the boreholes and reports on the soil and the water. A contractor with basement experience is worth involving early, since sequencing the dig drives the temporary works.

    We are chartered architectural technologists. We prepare the planning drawings, coordinate the specialists, assemble the assessment and check that its findings match the sheets. We do not replace the engineer, and our structural calculations page explains where that work fits.

    One coordinator matters because the documents must agree. A depth on the section that differs from the depth in the report is an easy reason for the council to ask for more information.

    Planning route, levy and cost

    Most basement proposals need planning permission, and the assessment goes in with the application. Your set of planning drawings carries the plans, sections and site plan that the report refers to.

    Basements have no automatic exemption from the Community Infrastructure Levy, and whether new basement floor area is chargeable depends on the charging council's rules. We note the question in the next-steps pack so it does not surprise you later.

    There is no list price because the scope depends on the site, the depth and the ground investigation the council expects. After the free constraints check we send a fixed quote, and if the council will not need an assessment we say so plainly.

    Why the borough matters more than the report

    Boroughs may also use Article 4 directions to bring smaller basements under planning control. Our council pages list each borough's published figures, such as conservation areas, listed buildings and the guidance in use, so you can see what applies to your street.

    Brent keeps a dedicated basement document, and Camden uses the Arup-based method described above. For the full drawing service behind an assessment, see our planning drawings.

    The national rules in figures

    Figures with their unit, period and source
    MeasureValuePeriodSource
    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
    Party Wall etc. Act 1996 s6(2): excavation within 6 m that meets a 45 degree plane drawn down from the bottom of the adjoining foundations6 m—legislation.gov.uk
    s6(5): notice at least one month before excavation; s6(6): notice accompanied by plans and sections showing site and depth of excavation1 month—legislation.gov.uk
    Explanatory booklet (12 May 2016): two months for work to an existing party structure (s2), one month for excavation (s6)2 months—gov.uk
    Booklet: no written response within 14 days of service means a dispute is deemed to have arisen (ss2 and 6); counter-notice within one month; county court appeal within 14 days of an award14 days—gov.uk
    Approved Document C (2004 ed. incl. 2010/2013) para 3.2: where the water table can rise to within 0.25 m of the lowest floor, drain by gravity or other effective means; para 3.4 groundwater stability; basement walls under groundwater pressure covered in the walls section0.25 m—gov.uk
    RBKC Basements SPD (April 2016) para 3.1 quoting Policy CL7(a): maximum 50% of each garden or open part of the site, measured separately for each garden (PDF p.12)50 %—rbkc.gov.uk
    RBKC Basements SPD (April 2016) para 9.2: CL7(j) minimum 1 m of soil above any part of the basement beneath a garden (PDF p.51)1 m—rbkc.gov.uk

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

    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

    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.
    Georgian brick terrace with sash windows, arched fanlight doorways, stone steps and iron railings in London
    The street around you
    Move across the plinth — the blocks rise around the pointer.

    Where we prepare basement impact assessments

    London

    Questions

    Do I need a basement impact assessment?

    Only if your council asks for one. National law does not require it, but boroughs write their own basement policies, and those decide when a report is needed. We check your address first.

    How much does a basement impact assessment cost?

    It depends on the site, the depth and the ground investigation your council expects. We give a fixed quote after a free constraints check, before you commit to anything.

    Does a basement need a party wall notice?

    Usually, if the dig is within 3 metres of a neighbour's building and deeper than its foundations, or within 6 metres on the 45-degree test. Serve the notice at least 1 month before work.

    Who writes the groundwater and stability sections?

    Specialists do: a ground investigation firm and a structural engineer. We coordinate their findings with the drawings so the council receives one consistent submission.

    Sources and dates

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    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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