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Hillingdon, London

Basement impact assessment in Hillingdon

Hillingdon has clear basement wording: a detailed technical statement on drainage, flooding, groundwater and structural stability for any new or extended basement, and a Local Plan policy that does not support basements spanning the full length or width of a property. We are chartered architectural technologists and give a 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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    Rows of brick terraced houses and gardens on a residential street

    Hillingdon's rules for a basement impact assessment

    The February local validation checklist lists a Basement Impact Assessment or Structural Survey among the requirements for full applications. Where a new or extended basement is proposed, the council wants a detailed technical statement assessing the effect on drainage, flooding, groundwater and structural stability. Without it, the application is not validated.

    The test it must meet is the basement policy in the Local Plan Part 2. Basement schemes must not harm drainage, run-off, structural stability or cumulative impacts. No habitable rooms are accepted in areas prone to flooding, and the policy does not accept basements in listed buildings or in conservation areas where they harm character. Basements running the full length or width of a property are not supported, and front and side lightwells are not acceptable.

    The landscaping rule is just as specific: suitable soil depth above the basement, typically the depth needed for shrubs, with a proportion of soft landscaping where the basement extends beyond the building footprint. The table on this page carries the citations. In practice, we draw the basement inside the house footprint with a rear lightwell if any, and put the soil-cover depth on the section before the report is written.

    Georgian brick terrace with sash windows, arched fanlight doorways, stone steps and iron railings in London

    Local figures for Hillingdon

    Figures with their unit, period and source
    MeasureValuePeriodSource
    A Basement Impact Assessment/Structural Survey is on the local requirement list for full applications; where a new or extended basement is proposed a detailed technical statement assessing impact on drainage, flooding, groundwater conditions and structural stability is required.Any new or extended basement—hillingdon.gov.uk, Hillingdon Local Planning Validation Checklist (February 2024), pdf p.5 (list); p.30, row 'Basement Impact Assessment/Structural Survey'
    Policy DMHD 3: basement schemes must not harm drainage, run-off, structural stability or cumulative impacts; no habitable rooms in areas prone to flooding; no basements in Listed Buildings or in conservation areas where they harm character. Basements extending the full length or width of a property are not supported; front and side lightwells not acceptable.No full-width/length basements—hillingdon.gov.uk, Hillingdon Local Plan Part 2 Development Management Policies (adopted January 2020), Policy DMHD 3 (p.136); paras A1.36, A1.39-A1.40, A1.43 (pp.134-135)
    Landscaping above basements must have suitable soil depth, typically 450 mm for shrubs, with a proportion of soft landscaping where the basement extends beyond the building footprint.450 mm soil for shrubs—hillingdon.gov.uk, Hillingdon Local Plan Part 2 Development Management Policies (adopted January 2020), paras A1.39 and A1.41 (pp.134-135)
    Article 4 directions150 Article 4 directions published by the council—planning.data.gov.uk
    Conservation areas33 named conservation areas published by the council—planning.data.gov.uk
    Listed building entries436 list entries (Historic England, within the LPA boundary)—planning.data.gov.uk
    Households living in a house68.9 % of households living in a house or bungalow (detached, semi-detached or terraced)Census 2021nomisweb.co.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

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

    Councils bordering Hillingdon
    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

    Hillingdon in numbers for a basement project

    Hillingdon records 150 Article 4 directions, names 33 conservation areas and holds 436 listed entries, with 45 on the Heritage at Risk Register. Green Belt covers 42 per cent of the borough.

    Around 68.9 per cent of households live in a house or bungalow and 56.6 per cent own their home. Gardens are 18.6 per cent of the land. The council took 1,147 householder decisions in the latest year, granting 90.2 per cent, 94.3 per cent in time.

    The Article 4 count is worth noting: where a direction covers your street, permitted development rights are removed and associated works such as front-boundary changes need an application.

    A common question is what happens to the soil and spoil. Excavating a basement removes a large volume of earth, and the construction route, lorry movements and any temporary support to the road or neighbours are all matters the council may raise. We note these on the drawings and in the cover note so the officer sees the construction has been thought through, though a full construction management plan is usually a condition rather than a validation item.

    Designing inside the Hillingdon limits

    The policy wording shapes the drawing from the first sketch. Keep the basement inside the footprint of the house rather than the full width, leave front and side elevations free of lightwells, and plan the rear garden so soft landscaping and suitable soil sit above any part that goes beyond the house. Doing so means the layout already answers the council's main concerns before the report is written.

    Flood-prone areas need an extra look because habitable rooms are not accepted there. We check the flood mapping first, and if your plot is affected we say so in the quote and explain what a lawful alternative might be, such as a plant or storage use.

    What the statement covers and who writes it

    It shows drainage, flooding, groundwater and structure are unharmed. A ground investigation firm drills and reports, a structural engineer designs the retaining walls, and we draw the plans and sections and coordinate the pack. Our structural calculations page explains the engineer's role.

    Approved Document C expects a drained floor where the water table can rise to within 0.25 metres of it.

    Party walls, planning route and cost in Hillingdon

    Under the Party Wall etc. Act 1996, excavation within 3 metres of an adjoining building and below its foundations needs a notice, as does a dig within 6 metres on the 45-degree test. Our party wall notices service prepares the plans.

    Planning drawings come from our planning drawings work, and the borough's figures are on architectural drawings in Hillingdon. Ealing and Harrow border Hillingdon and write their own rules.

    The service is unpriced. After the free constraints check you receive a fixed quote.

    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.
    Brick cottage with a tiled roof, chimneys and a front dormer window
    The street around you
    Move across the plinth — the blocks rise around the pointer.

    Other services in Hillingdon

    Basement impact assessments nearby

    Questions

    Can I build a full-width basement in Hillingdon?

    The Local Plan policy does not support basements extending the full length or width of a property, so we draw within the house footprint.

    Are lightwells allowed in Hillingdon?

    Front and side lightwells are not acceptable under the policy. A rear one may be considered.

    What soil cover is expected over a basement?

    Suitable soil depth, typically the depth shrubs need, with soft landscaping where the basement goes beyond the house footprint.

    Does Hillingdon accept a basement in a conservation area?

    Not where it harms the character of the area. The borough names several conservation areas, so we check whether your street is inside one before drawing the front of the house.

    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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