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Bexley, London

SAP calculations in Bexley

SAP calculations in Bexley show building control that an extension or a new home meets the energy rules in Approved Document L. We prepare the figures from your drawings, with a fixed quote after a free constraints check, VAT included.

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 sap calculations

    Five 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. 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.

    4. Building regulations

      Technical drawings are coordinated with the structural engineer's calculations, the drainage and the planning conditions.

    5. Approval and hand-over

      Building control approval, then a next-steps pack covering the commencement notice, party wall timing and CIL.

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

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    House extension under construction with scaffolding, new glazed doors and building materials

    SAP calculations in Bexley's suburban housing

    Bexley is a borough of houses. Some 75.3 per cent of households live in a house or bungalow and 68.6 per cent own their home, so most of our enquiries here are owners extending a semi-detached or terraced house, not developers building blocks of flats. That makes the extension rules the ones that matter most.

    The council's design guidance for such projects is Part 2 of its guide, the Design Guide SPD Part 2: Building Alterations and Extensions, read alongside Design Guide SPD Part 1: Design Principles. These govern the look and size of what you add. The energy limits sit outside them, so a design can satisfy the council's guide and still need a calculation to satisfy building control.

    A typical Bexley project is a single-storey rear extension with a sliding door set, perhaps with a rooflight. It is a small building with a lot of glass for its floor area, which is why we count the openings on day one rather than after the builder has ordered them.

    Pencil resting on a dimensioned technical drawing beside a scale rule

    Local figures for Bexley

    Figures with their unit, period and source
    MeasureValuePeriodSource
    Households living in a house75.3 % of households living in a house or bungalow (detached, semi-detached or terraced)Census 2021nomisweb.co.uk
    Owner-occupied households68.6 % of households owner-occupied (owned outright or with mortgage/loan)Census 2021nomisweb.co.uk
    Householder applications granted94.6 %1 July 2025 - 30 June 2026assets.publishing.service.gov.uk
    Conservation areas23 named conservation areas published by the council—planning.data.gov.uk
    Listed building entries114 list entries (Historic England, within the LPA boundary)—planning.data.gov.uk
    Mayoral CIL rate (indexed)30.3 £/m² (indexed, permissions granted in 2026)—london.gov.uk
    Approved Document L Vol 1 2021 incl. 2023 amendments, Table 4.2New elements in existing dwellings, limiting U-values W/(m²·K): roof 0.15, wall 0.18, floor 0.18, windows 1.4 (or WER Band B), doors 1.4—gov.uk
    Approved Document L Vol 1 2021, para 10.7(d)Windows, roof windows, rooflights and doors in an extension should not exceed 25% of the extension floor area plus the area of openings removed or covered—gov.uk
    AD L Vol 1 2026, regulation 24 intention (p.11)For new dwellings AD L 2026 names two methods: the Home Energy Model (HEM) and SAP; validity dates are in the Notice of Approval—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 Bexley
    Building regulations, layer by layerPart A: structure: Approved Document A: Structure, 2004 edition incorporating 2004, 2010 and 2013 amendments. Part B: fire safety: All dwellings: fire detection and alarm system minimum Grade D2 Category LD3 (BS 5839-6), mains-operated smoke alarms to BS EN 14604. Part L: insulation: New elements in existing dwellings, limiting U-values W/(m²·K): roof 0.15, wall 0.18, floor 0.18, windows 1.4 (or WER Band B), doors 1.4. Part K: stairs: 2.0 m clear headroom over stairs. Full plans application: Building control body must decide a full plans application within 5 weeks, or longer if agreed in writingPart A: structurePart B: fire safetyPart L: insulationPart K: stairsFull plans applicationABCDE

    Building regulations, layer by layer

    The Approved Documents that shape most house projects, from structure to the application itself.

    A · Part A: structure
    Approved Document A: Structure, 2004 edition incorporating 2004, 2010 and 2013 amendments
    B · Part B: fire safety
    All dwellings: fire detection and alarm system minimum Grade D2 Category LD3 (BS 5839-6), mains-operated smoke alarms to BS EN 14604
    C · Part L: insulation
    New elements in existing dwellings, limiting U-values W/(m²·K): roof 0.15, wall 0.18, floor 0.18, windows 1.4 (or WER Band B), doors 1.4
    D · Part K: stairs
    2.0 m clear headroom over stairs
    E · Full plans application
    Building control body must decide a full plans application within 5 weeks, or longer if agreed in writing

    Which route fits a Bexley extension

    The simplest route is to meet the individual limits, set as: New elements in existing dwellings, limiting U-values W/(m²·K): roof 0.15, wall 0.18, floor 0.18, windows 1.4 (or WER Band B), doors 1.4. If your walls, roof and floor reach those figures and your windows are modest, no calculation is needed.

    The glazing rule is the tripwire. It says: Windows, roof windows, rooflights and doors in an extension should not exceed 25% of the extension floor area plus the area of openings removed or covered. Rooflights count toward it as well as doors, so a lantern above bifold doors can tip a small extension over the line.

    If it does, the guidance offers: Alternative routes: area-weighted U-value (para 10.9) or a SAP calculation against a notional extension (para 10.10). A SAP comparison against a notional extension is the more flexible option, and is the one we run when the client wants the glazing kept.

    Planning and heritage context in Bexley

    Bexley decided 877 householder applications in its latest year and granted 94.6 per cent, with 99.4 per cent in time. That is a high grant rate, but it describes the council's caseload, not your house, so the energy design still needs to be right first time.

    The borough has 23 conservation areas and 114 listed building entries, including 6 at Grade I. Where a home is listed or in a conservation area, windows and roof coverings are examined closely, and the SAP assumptions have to match what is actually consented.

    Prior approval for larger home extensions is available for some houses; the council granted 7 and refused 8 in the latest national return. See planning drawings in Bexley for how we choose between that, a certificate and a householder application.

    New houses and the changing method

    A new dwelling uses a different method. The guidance says: For new dwellings AD L 2026 names two methods: the Home Energy Model (HEM) and SAP; validity dates are in the Notice of Approval. If you are building a house in a garden plot or replacing an existing home, your assessor needs to know which edition will apply.

    The next edition begins as follows: AD L 2026 takes effect 24 Mar 2027 (24 Sep 2027 for higher-risk building work), and the transition rule is AD L/F 2021 continue where plans/notices are deposited before 24 Mar 2027 and work starts before 24 Mar 2028. We record both dates in the programme we give you.

    Because Bexley has a large stock of family houses, some owners combine an extension with a loft conversion. The energy route is then chosen for the whole project, and our loft conversion drawings in Bexley are prepared with that in mind.

    Cost, speed and the levies around the work

    You get a fixed quote after a free constraints check, VAT included, before any work starts. The assessor works from our model, so the floor areas and window sizes match the drawings, and the results are ready for the submission described in building regulations drawings in Bexley.

    Floor area added also affects levies. The borough's schedule is the London Borough of Bexley Community Infrastructure Levy Charging Schedule April 2015, at £40/m² (Zone 1); £60/m² (Zone 2), plus indexation. The Mayoral rate in this borough is £30.3 per square metre at the current indexed rate. We note the area on the drawings so you can check the figures early.

    Prices in the borough average £413,343. Energy spending is a small part of an extension's cost, and it is a cheaper problem on paper than on site. See architectural drawings in Bexley for everything else we prepare here.

    What you receive

    Building regulations section drawing showing the construction build-up
    Sample drawing. Building regulations section showing construction build-up for a building control check.
    Red-brick English house with rows of solar panels on its tiled roof
    Energy in
    Move the pointer up — the plug rises toward the socket.

    Other services in Bexley

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    Questions

    Which Bexley guide applies to a house extension?

    Part 2 of the council's design guide: the Design Guide SPD Part 2: Building Alterations and Extensions. Energy limits come separately from Approved Document L.

    Do rooflights count toward the glazing limit?

    Yes. The guidance says: Windows, roof windows, rooflights and doors in an extension should not exceed 25% of the extension floor area plus the area of openings removed or covered.

    Will a new home in Bexley need a SAP calculation?

    A new dwelling is assessed by a calculation method, as set out in For new dwellings AD L 2026 names two methods: the Home Energy Model (HEM) and SAP; validity dates are in the Notice of Approval.

    How much does a SAP calculation cost?

    Fixed quote after a free constraints check, VAT included.

    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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