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Westminster, London

SAP calculations in Westminster

SAP calculations in Westminster give building control the energy evidence for mews houses, roof additions, flats and new homes in a borough where heritage rules shape almost every elevation. 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 Westminster: roofs, mews and listed streets

    Westminster has two guides that matter most to an energy design. The first is Roofs: a guide to alterations and extensions on domestic buildings SPG, which covers how roofs may be altered and extended. The second is Mews: a guide to alterations SPG, for the converted stables and coach houses behind the grand terraces. Both sit beneath the City Plan 2019-2040 (2026).

    A roof addition is the clearest example of why the SAP and the planning design must be reconciled. A thicker roof build-up, needed to meet the energy limits, can lift the roofline by a few centimetres, and in a street where the roof profile is protected that can decide whether a scheme is acceptable.

    In a mews, the problem is reversed. Walls are often shared or close to neighbours, plots are tiny, and every centimetre of insulation comes out of internal space. We test the build-up against the room dimensions before the drawings are issued.

    Pencil resting on a dimensioned technical drawing beside a scale rule

    Local figures for Westminster

    Figures with their unit, period and source
    MeasureValuePeriodSource
    Conservation areas56 named conservation areas published by the council—planning.data.gov.uk
    Article 4 directions10 Article 4 directions published by the council—planning.data.gov.uk
    Listed building entries4,015 list entries (Historic England, within the LPA boundary)—planning.data.gov.uk
    Grade I listed entries205 list entries—planning.data.gov.uk
    Householder applications decided in time79 % decided within 8 weeks or an agreed extended period1 July 2025 - 30 June 2026assets.publishing.service.gov.uk
    Average price, flat775,956 GBP2026-07publicdata.landregistry.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
    Approved Document O 2021, para 0.3Approved Document O (overheating) applies to new residential buildings—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 Westminster
    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

    What an alteration must achieve

    The limits for new elements in an existing building are: 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. For a new roof, wall or floor those figures are straightforward to reach with modern insulation.

    Glazing is capped on the simple route: 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. Mews houses and roof pavilions often use glass heavily, because it brings in light and cuts mass, and so reach the cap quickly.

    The alternatives are: Alternative routes: area-weighted U-value (para 10.9) or a SAP calculation against a notional extension (para 10.10). For a heavily glazed rooftop room, we normally run the SAP comparison, which allows a strong roof and floor to balance the glass.

    Westminster's heritage density

    Westminster has 56 conservation areas and 10 Article 4 directions, and Historic England lists 4,015 entries here, including 205 at Grade I and 1 World Heritage Site. Any energy upgrade to a visible element needs thought.

    Householder statistics are modest in volume. The council decided 366 householder applications in its latest year, granting 82 per cent, with only 79 per cent decided in time. The delay is worth building into a programme, and our planning drawings in Westminster are prepared with it in mind.

    Where permitted development applies, a lawful development certificate in Westminster records it; where it does not, the energy route is chosen before the application goes in.

    New homes and the dates that matter

    A newly built dwelling follows a separate path. 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. The next edition starts as follows: AD L 2026 takes effect 24 Mar 2027 (24 Sep 2027 for higher-risk building work).

    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 note the deposit date and the start date on the programme from the outset.

    Overheating is a real issue in a central setting. The rule says that Approved Document O (overheating) applies to new residential buildings. A top-floor flat with large windows needs shading and ventilation planned in, and a new home needs a certificate at completion: An energy performance certificate is required on construction of a new dwelling.

    Price, levy and the rest of the set

    You get a fixed quote after a free constraints check, VAT included. The assessor works from our model, so the figures match your drawings, and the results support the submission in building regulations drawings in Westminster.

    Added floor area carries a levy. The borough's schedule is the Westminster CIL Charging Schedule, at £811.80/m² (Prime); £590.40/m² (Core); £295.20/m² (Fringe) [schedule rates £550, £400, £200]. The Mayoral rate is £96.97 per square metre, indexed.

    Flat prices average £775,956, and the market has moved -20.7 per cent over twelve months. See architectural drawings in Westminster for the full range, or structural calculations in Westminster for the beams that go with a roof addition.

    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 Westminster

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    Questions

    Does a Westminster roof extension need a SAP calculation?

    Not always. If the new roof meets the elemental limits and the glazing stays within its cap, no. A glass-heavy roof room often does, because the calculation lets the design balance glass against insulation.

    Which Westminster guide covers roof alterations?

    The Roofs: a guide to alterations and extensions on domestic buildings SPG. For mews houses, it is the Mews: a guide to alterations SPG.

    Is overheating considered?

    For new homes, yes: Approved Document O (overheating) applies to new residential buildings.

    How much does it cost in Westminster?

    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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