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Croydon, London

Basement impact assessment in Croydon

In Croydon a basement impact assessment is requested where the site sits at risk of groundwater flooding, and it travels inside the Flood Risk Assessment rather than as a separate report. We are chartered architectural technologists: we check your address against the borough's trigger, coordinate the specialists 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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    Brick cottage with a tiled roof, chimneys and a front dormer window

    Croydon's rule for a basement impact assessment

    Croydon does not publish a basement supplementary document with depth limits or garden percentages. What it does publish is a trigger tied to water. Its Local Validation List says that where a basement is proposed in an area at risk of groundwater flooding, the Flood Risk Assessment must carry a basement impact assessment. That assessment locates the scheme against aquifers and watercourses, explains the flooding and drainage effects, sets out measures that protect your house and the neighbours, and describes the drainage and maintenance that will follow.

    The adopted Croydon Local Plan 2018 says the same thing from the policy side. Where a site is at risk of groundwater flooding, the council will ask for the assessment as part of the Flood Risk Assessment for any application with a basement or lower ground floor, informed by ground investigations. It also expects new basements that connect to the sewer to have a positive pumped device, so a gravity connection that relies on the road sewer being empty is not enough.

    What this means for the design: the question to settle first is not how deep you can go but whether your plot is flagged for groundwater. If it is, the ground investigation comes before the planning drawings are finalised, because the borehole readings feed both the report and the section drawing. If it is not, a lighter structural statement may be all the officer needs, and we say so in the quote rather than selling you a report the council will not read.

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

    Local figures for Croydon

    Figures with their unit, period and source
    MeasureValuePeriodSource
    If basement development is proposed in an area at risk of groundwater flooding, the Flood Risk Assessment must include a basement impact assessment locating the development against aquifers or watercourses, explaining flooding and drainage impacts, measures to reduce flood risk to the proposed and neighbouring properties, and ongoing drainage and maintenance. No Croydon basement SPD with depth or garden limits was found.BIA inside FRA where groundwater flood risk—croydon.gov.uk, Croydon Local Validation List (May 2026), PDF p.15 (printed 13), 'Plans and Drawings'
    Local Plan para 8.34: where a site is at risk of groundwater flooding, the Council will request a Basement Impact Assessment as part of the Flood Risk Assessment for any application with basement or lower ground floor development, informed by ground investigations; new basements connected to the sewer must have a positive pumped device.BIA informed by ground investigation; pumped device—croydon.gov.uk, Croydon Local Plan 2018 (revised 2024, updated), para 8.34, para 8.34, printed p.174 (PDF p.189)
    Domestic gardens share of land (GLUD 2005)32.8 % of land that is domestic garden (GLUD 2005)2005data.london.gov.uk
    Households living in a house60.2 % of households living in a house or bungalow (detached, semi-detached or terraced)Census 2021nomisweb.co.uk
    Householder applications granted86.6 %1 July 2025 - 30 June 2026assets.publishing.service.gov.uk
    Conservation areas19 named conservation areas published by the council—planning.data.gov.uk
    Listed building entries180 list entries (Historic England, within the LPA boundary)—planning.data.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

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

    Councils bordering Croydon
    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

    Croydon in numbers for a dig-down project

    Domestic gardens make up 32.8 per cent of the land in the borough, which is why so many basement enquiries here are rear-garden schemes under semi-detached and terraced houses. Around 60.2 per cent of households live in a house or bungalow, and the rest are in flats where a basement is rarely an option.

    On the planning side, Croydon decided 786 householder applications in the latest year we hold and granted 86.6 per cent of them. Basement schemes are usually judged as householder or minor proposals, so that figure is a fair backdrop, not a prediction. The figures to watch are heritage ones: the council names 19 conservation areas and holds 180 listed building entries, and a basement beneath or beside either brings a heritage question as well as a structural one.

    The table below lists the figures with their sources and dates. We do not round or restate them in the copy, so what you read there is what the dataset holds.

    Sequence and timing for a Croydon basement

    Order matters more than most owners expect. First comes the flood and groundwater screen, which needs only a desk study and the council's own maps. Then the ground investigation: boreholes and standpipes, with readings taken over several weeks so the water level is not a one-day snapshot. Only then is the structural design fixed, because the engineer sizes the retaining walls and the pumped drainage from those readings.

    We keep the planning drawings in draft until that point. A section drawn before the readings arrive can easily show a floor level that the groundwater result later rules out, and redrawing a submitted set costs far more than waiting a fortnight. If the screen shows no groundwater concern, the whole sequence shortens, and we tell you that in the quote.

    What the assessment covers once it is triggered

    Think of it as three questions the officer needs answered on paper. Will the ground and the neighbouring foundations stay stable? Will the new box interrupt groundwater flow? Will surface water and sewer flooding be made worse for you or anyone next door? The groundwater trigger in Croydon puts most weight on the second and third.

    Building Regulations add their own test. Approved Document C asks that, where the water table can rise to within 0.25 metres of the lowest floor, the floor is drained by gravity or other effective means. A basement that satisfies the planning officer but not the building control surveyor still has to be redesigned, which is why we keep one set of levels across both submissions.

    A structural engineer designs the retaining walls and slab, and a ground investigation firm drills and reports. We assemble their findings, draw the plans and sections the report cites and keep the figures consistent across the pack. Our structural calculations page explains where the engineer's work starts and ours stops.

    Neighbours, party walls and the next steps

    Digging near a neighbour's wall is a separate legal matter from planning. Under the Party Wall etc. Act 1996 you must serve notice where the excavation is within 3 metres of an adjoining building and goes below its foundations, and the same applies within 6 metres where a 45-degree line from the bottom of those foundations meets your dig. Our party wall notices service prepares the drawings that go with the notice.

    Most Croydon basements need planning permission, and the report goes in with the application. Your plans, sections and site plan come from our planning drawings work. For the wider picture of what the borough publishes about its housing stock and heritage, see architectural drawings in Croydon.

    Neighbouring boroughs write their rules differently. Lambeth and Merton border Croydon, and a plot near the boundary is decided by whichever council the address falls in. We check that first, because the validation list and the local plan differ from one borough to the next.

    Cost and what you receive

    This is an unpriced service because the scope depends on depth, ground and flood status. After the free constraints check we send a fixed quote, name what is included and say plainly if Croydon is unlikely to ask for an assessment on your site. You receive the coordinated drawings, the assessment pack built from the specialists' reports and a plain-English note of the council's likely questions.

    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.
    Pencil resting on a dimensioned technical drawing beside a scale rule
    The street around you
    Move across the plinth — the blocks rise around the pointer.

    Other services in Croydon

    Basement impact assessments nearby

    Questions

    When does Croydon ask for a basement impact assessment?

    Where a basement is proposed on a site at risk of groundwater flooding. The assessment sits inside the Flood Risk Assessment and is informed by ground investigation. We check whether your address is flagged before any report is commissioned.

    Does Croydon limit how deep or wide a basement can be?

    We found no Croydon basement document with depth or garden-percentage limits. Other tests still apply: heritage, trees, neighbours and flood risk, so the scheme is judged on its merits.

    Does a Croydon basement need a pump?

    The Local Plan expects new basements connected to the sewer to have a positive pumped device. Approved Document H also looks at surcharge risk, so the drainage design must satisfy both.

    Who prepares the groundwater findings?

    A ground investigation firm and a structural engineer, not us. We coordinate their work 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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