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Harrow, London

SAP calculations in Harrow

Most Harrow households live in houses, and a SAP calculation here often accompanies a large rear extension, a loft or a replacement home, and we coordinate it with an assessor from your drawings for a fixed quote after a free constraints check, VAT included. With 31 conservation areas and 295 listed entries, we check the designations first.

Fixed quote after a free constraints check

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  • Fixed fees, VAT included

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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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    Pencil resting on a dimensioned technical drawing beside a scale rule

    SAP calculations in Harrow and the Residential Design Guide

    The council's Residential Design Guide SPD is the document that governs how a house may be extended, and it sets the footprint before any energy figure exists.

    A bigger extension means more wall, more roof and usually more glass, which is where a SAP model starts to earn its fee. We take the permitted envelope from the guide and calculate the largest version of the design that planning can accept, so the energy figures cover the real building.

    Red-brick English house with rows of solar panels on its tiled roof

    Local figures for Harrow

    Figures with their unit, period and source
    MeasureValuePeriodSource
    Listed building entries295 list entries (Historic England, within the LPA boundary)—planning.data.gov.uk
    Conservation areas31 named conservation areas published by the council—planning.data.gov.uk
    Grade II* listed entries20 list entries—planning.data.gov.uk
    Heritage at Risk entries22 Heritage at Risk Register entries within the LPA boundary (Historic England)—planning.data.gov.uk
    Green Belt share of the council area22 % of council area covered by green-belt polygons—planning.data.gov.uk
    Households living in a house65.4 % of households living in a house or bungalow (detached, semi-detached or terraced)Census 2021nomisweb.co.uk
    Average price, detached1,206,272 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

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

    Councils bordering Harrow
    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

    Suburban plots, Green Belt and open land

    Around 65.4 per cent of Harrow households live in a house, and 58.8 per cent own it. About 22 per cent of the borough is covered by Green Belt polygons, and 9 scheduled monuments touch the area.

    Near the Green Belt, planning restricts the size of additions, so the floor area is fixed early and the calculation is built on it. Elsewhere in the borough the design guide and the plot set the limit.

    Heritage entries and conservation areas

    Historic England lists 295 entries in the borough, of which 20 are Grade II*, and 22 appear on the Heritage at Risk Register. The borough also has 31 conservation areas.

    For a house in a conservation area, the council may limit window style and the choice of external finish. We confirm those limits before choosing the glass specification, because a thick triple-glazed unit that planning refuses is no use to anyone.

    Prices and what the budget allows

    A detached house averaged £1,206,272 and a terraced one £536,748 in the latest Land Registry data. The detached price supports a high specification, and many owners ask what a heat pump or a thicker roof would add to the rating.

    We answer with the model. A single run shows the result under the elemental limits, then again with upgrades, so the owner decides on figures and not on a sales pitch.

    Rules for extensions and new homes

    The limits for new elements 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 separate: 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.

    When a large rear room breaks the cap, the guidance gives the alternatives: Alternative routes: area-weighted U-value (para 10.9) or a SAP calculation against a notional extension (para 10.10). A replacement or infill dwelling is different: For new dwellings AD L 2026 names two methods: the Home Energy Model (HEM) and SAP; validity dates are in the Notice of Approval.

    Our house extension drawings in Harrow put the glazing count on the plan, and the assessor reads it from there.

    Energy design for a Harrow family house

    Some Harrow projects add a deep ground-floor room and a loft in one go. That is a lot of new fabric at once, and the energy model covers both together. The loft roof, the dormer walls and the new rear walls are each assessed against their own limit.

    Heat loss through the old part of the house is unaffected, but the new rooms must not make the whole house worse. We check how the new heating zones interact with the existing system, because an undersized boiler can be an unwelcome surprise.

    We also look at ventilation. A larger, better-sealed house needs extract fans and trickle vents sized correctly, and the drawings show them, so the builder fits what the calculation assumed.

    Owners sometimes ask whether solar panels count. They can, as part of the calculation for a new dwelling, and we include them if you intend to fit them, since they change the result and the roof layout together.

    Planning the Harrow package

    See architectural drawings in Harrow for the local constraints and building regulations drawings in Harrow for the construction detail. Deep rear additions also need the beams in structural calculations in Harrow.

    Price is a fixed quote after a free constraints check, VAT included. Send the address and the brief.

    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.
    House extension under construction with scaffolding, new glazed doors and building materials
    Energy in
    Move the pointer up — the plug rises toward the socket.

    Other services in Harrow

    SAP calculations nearby

    Questions

    Does a big rear extension in Harrow need SAP?

    Not necessarily, but a deep room with a glass wall often exceeds the glazing cap. 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.

    Does Green Belt change the energy rules?

    No. It limits size through planning. About 22 per cent of Harrow is Green Belt, and the energy rules are the same everywhere.

    What energy edition applies to a new home?

    AD L/F 2021 continue where plans/notices are deposited before 24 Mar 2027 and work starts before 24 Mar 2028.

    What does it cost in Harrow?

    Fixed quote after a free constraints check, VAT included.

    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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