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Islington, London

Basement impact assessment in Islington

A basement impact assessment in Islington has to show that a new lower floor meets the borough's own geometry limits, which cap how much garden you can dig under, how deep you can go and how much soil must stay on top. Terraced streets leave little room, so we test the numbers before design starts and give 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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    Georgian brick terrace with sash windows, arched fanlight doorways, stone steps and iron railings in London

    Islington's basement limits for an assessment

    Islington writes its basement test into the Islington Local Plan: Strategic and Development Management Policies, Site Allocations, and Bunhill and Clerkenwell Area Action Plan, adopted in 2,023, and expands it in the Basement Development SPD. The headline rule is: Under 50% garden; 1 storey; 3 m. It is measured separately for each garden or unbuilt area, so a house with a rear yard and a side passage is tested twice.

    The second rule is about what lies above the box. In the borough's own words the requirement is 1 m soil + 200 mm drainage, with a thicker layer where the site is constrained or large trees are planned. A basement that squeezes under a rear extension and leaves no room for that soil fails before the engineer is involved.

    These are the council's figures, not a London-wide standard. We list them first because they decide whether an assessment is worth commissioning: a scheme that cannot meet the geometry limits needs redesigning, not reporting.

    Brick cottage with a tiled roof, chimneys and a front dormer window

    Local figures for Islington

    Figures with their unit, period and source
    MeasureValuePeriodSource
    Policy DH4 D: residential basements must cumulatively occupy less than 50% of the original garden/unbuilt area, or the part outside the host footprint must be no more than 50% of the host building footprint (whichever is lesser); no more than one storey below the lowest original habitable floor; maximum 3 m floor-to-ceiling height; must not significantly alter the prevailing garden level. Measured separately for each garden or unbuilt area.Under 50% garden; 1 storey; 3 m—islington.gov.uk, Islington Local Plan: Strategic and Development Management Policies (adopted 28 September 2023), Policy DH4 D(i)-(vi), p.264; paras 8.66-8.67, p.265
    Para 8.74: unless within 3 m of the original rear wall, any basement beyond the original footprint must include at least 1 m of soil plus 200 mm drainage above the basement top cover; up to 1.5 m of soil for constrained sites or where large trees are proposed. A basement under an existing non-original basement making two storeys will be resisted.1 m soil + 200 mm drainage—islington.gov.uk, Islington Local Plan: Strategic and Development Management Policies (adopted 28 September 2023), para 8.74, p.266; para 8.67, p.265
    Conservation areas41 named conservation areas published by the council—planning.data.gov.uk
    Listed building entries1,050 list entries (Historic England, within the LPA boundary)—planning.data.gov.uk
    Grade I listed entries12 list entries—planning.data.gov.uk
    Domestic gardens share of land (GLUD 2005)19.7 % of land that is domestic garden (GLUD 2005)2005data.london.gov.uk
    Households living in a house16.3 % of households living in a house or bungalow (detached, semi-detached or terraced)Census 2021nomisweb.co.uk
    Householder applications granted92.6 %1 July 2025 - 30 June 2026assets.publishing.service.gov.uk
    Householder applications decided in time98.8 % decided within 8 weeks or an agreed extended period1 July 2025 - 30 June 2026assets.publishing.service.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 Islington
    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

    Why Islington basements are tested hard

    Islington is dense. Only 16.3 per cent of households live in a house, and just 26.7 per cent own their home, so many basement jobs are for freeholders of narrow terraces or for owners of maisonettes who need neighbours' consent as well as the council's.

    Gardens are scarce too. Domestic gardens cover 19.7 per cent of the borough's land in the national land-use database, and the limit on garden coverage therefore bites on almost every plot. We measure the original garden from the oldest reliable survey we can find, not from the present plot, because later extensions do not enlarge your allowance.

    Water and ground behave differently from street to street. The assessment still has to deal with the same three themes: stability of the ground and the neighbouring walls, groundwater, and surface water or flood risk. We commission the ground investigation early so the findings can shape the design instead of undoing it.

    Heritage around a basement in Islington

    Islington has 41 conservation areas and 1,050 listed building entries, with 12 at Grade I and 34 at Grade II*. Most terraced basements sit inside a conservation area, so a lightwell on the front elevation is judged on appearance as well as depth.

    That matters because the front is what the street sees. A railing line, a stair down to an area and a replaced window can each be refused even when the engineering is sound. We draw the front elevation with the same care as the section.

    How Islington decides applications that include a basement

    In its latest published year the council decided 1,426 planning applications. It granted 92.6 per cent of householder applications and 86.5 per cent of minor ones, and 98.8 per cent of householder decisions came within the statutory period or an agreed extension. Those figures cover every project type, so they do not predict the outcome of a basement, but they show a council that works to time and checks detail.

    The set of planning drawings in Islington we prepare carries the plans, sections and site plan the report points to. If you are also thinking about a rear extension, our structural calculations in Islington page explains where the engineer's work meets ours.

    Party wall duties before you dig in Islington

    With houses this close together, the Party Wall etc. Act 1996 applies to most basements. A notice is needed for excavation within 3 metres of an adjoining building that goes deeper than its foundations, and within 6 metres on the diagonal test. Serve it at least 1 month before work.

    A neighbour who does not reply in writing within 14 days is treated as being in dispute. We prepare the plans for the notice through our party wall notices service.

    Fixed quote for an Islington assessment

    We are chartered architectural technologists (MCIAT). We coordinate the engineer, the ground investigation and the drawings into one submission. The price depends on depth, soil and the number of gardens involved, so we give a fixed quote once the constraints check is done, and we tell you plainly if the geometry limits rule the scheme out.

    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.
    Rear view of brick terraced houses in London with slate roofs and chimney stacks along the shared roofline
    The street around you
    Move across the plinth — the blocks rise around the pointer.

    Other services in Islington

    Basement impact assessments nearby

    Questions

    How much of my garden can an Islington basement take?

    The council's test is Under 50% garden; 1 storey; 3 m. It is measured separately for each garden or unbuilt area.

    How much soil must sit above an Islington basement?

    The borough asks for 1 m soil + 200 mm drainage above the basement beyond the original footprint, and more on constrained sites.

    Does a conservation area change an Islington basement?

    Yes. With 41 conservation areas in the borough, the lightwell and front elevation are judged on appearance as well as engineering.

    What does an Islington basement impact assessment cost?

    It depends on depth, soil and the number of gardens. You receive a fixed quote after a free constraints check.

    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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