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Merton, London

SAP calculations in Merton

A SAP calculation in Merton is the energy assessment building control may ask for when a house is extended or a new home is built. We coordinate it with an assessor and your drawings, for a fixed quote after a free constraints check. Merton houses range from Wimbledon villas to Mitcham terraces, and the right energy route depends on which you have.

Fixed quote after a free constraints check

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We use it to check conservation areas and constraints.
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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

    Merton's guidance and what it asks of an energy design

    Merton applies the Merton Local Plan 2024, adopted in 2,024, with the Borough Character Study SPD and the Small sites toolkit SPD for small plots. A Basement and subterranean SPD covers cellars and underground work. They are planning documents, so they judge character, scale and structure, not thermal performance.

    We read them to learn what shape of building is likely to be approved, and then work out what that shape needs to pass the energy rules. If the council wants a modest rear extension, the energy design is a modest one. If the guidance points to a larger scheme on a bigger plot, the sums grow with it, and we plan the calculation to match.

    Pencil resting on a dimensioned technical drawing beside a scale rule

    Local figures for Merton

    Figures with their unit, period and source
    MeasureValuePeriodSource
    Households living in a house60.4 % of households living in a house or bungalow (detached, semi-detached or terraced)Census 2021nomisweb.co.uk
    Owner-occupied households55.6 % of households owner-occupied (owned outright or with mortgage/loan)Census 2021nomisweb.co.uk
    Average price, flat405,123 GBP2026-07publicdata.landregistry.gov.uk
    Average price, detached2,338,085 GBP2026-07publicdata.landregistry.gov.uk
    Householder applications granted92.9 %1 July 2025 - 30 June 2026assets.publishing.service.gov.uk
    Householder applications decided in time85.2 % decided within 8 weeks or an agreed extended period1 July 2025 - 30 June 2026assets.publishing.service.gov.uk
    Larger home extension prior approvals refused80 decisions2025 Q2 to 2026 Q1 (1 Apr 2025 to 31 Mar 2026)assets.publishing.service.gov.uk
    Conservation areas28 named conservation areas published by the council—planning.data.gov.uk
    Listed building entries243 list entries (Historic England, within the LPA boundary)—planning.data.gov.uk
    Heritage at Risk entries23 Heritage at Risk Register entries within the LPA boundary (Historic England)—planning.data.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 Merton
    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

    Two kinds of Merton house

    60.4 per cent of Merton's households live in a house, and the owner-occupied share is 55.6 per cent. The price range is wide: a flat averaged £405,123 in the latest Land Registry figures, a semi-detached house £954,064 and a detached house £2,338,085.

    At the upper end, detached houses are often extended on a large scale, with glass rear elevations and basement additions. At the lower end, a terraced house gains a modest kitchen box. The first usually needs a SAP calculation. The second often does not, and we would rather tell you that than sell you a report you can do without.

    How the elemental route works in Merton

    For a new part of an existing home the limits 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.

    The glazing rule is the one most designs trip on: 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. Where a design exceeds it, a calculation can compare it with a notional extension, as described here: Alternative routes: area-weighted U-value (para 10.9) or a SAP calculation against a notional extension (para 10.10).

    A glazed extension that cannot pass on simple limits is not a problem if the calculation shows that the whole building performs well. We run it before the planning application, so the drawings reflect what will work. Our house extension drawings in Merton are prepared that way.

    Merton's decisions, and the one figure to watch

    Merton decided 759 householder applications in its latest published year and granted 92.9 per cent. It decided only 85.2 per cent within eight weeks or an agreed extension, so allow time in your plan. A late decision can push back the date on which your plans are deposited, and with the standard changing over time that date can matter.

    Larger home extensions under prior approval met a harder test. The council granted 22 and refused 80 in the latest year.

    Heritage and constraints around Wimbledon and beyond

    Merton has 28 conservation areas and 243 list entries. 23 entries appear on the Heritage at Risk Register. Where a building is protected, the energy design adapts: Approved Document L gives historic buildings special consideration, so we agree an approach before the design is fixed.

    Our free check also covers Article 4 directions, trees and flood risk, and we write the result down in plain English, so that the quote reflects the site and not only the plan. We tell you early if one of them changes the route, rather than after the drawings are drawn.

    New homes and the changing standard

    New homes on Merton's small plots follow the dwelling method: 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 dates are: AD L 2026 takes effect 24 Mar 2027 (24 Sep 2027 for higher-risk building work). Our building regulations drawings in Merton carry the specification, and the overview is on architectural drawings in Merton. Neighbouring pages: SAP calculations in Sutton and SAP calculations in Wandsworth.

    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 Merton

    SAP calculations nearby

    Questions

    Does a Wimbledon extension need a SAP calculation?

    It depends on the glazing and the fabric. Large glazed rear walls often do, and modest extensions often do not.

    Do Merton's SPDs set energy requirements?

    They are planning documents on design and character. Energy compliance is a Building Regulations matter.

    How long does Merton take to decide householder applications?

    In the latest year 85.2 per cent were decided within eight weeks or an agreed extension.

    How much do SAP calculations cost in Merton?

    We give a fixed quote after a free constraints check, because the price depends on the building and the design.

    Sources and dates

    City of London skyline glowing at dusk across the Thames under a dramatic orange sky

    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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