Summary
- The 2021 edition of Approved Document L (in force June 2022) cut new-dwelling carbon targets by ~31%, introduced SAP 10, tightened limiting U-values, and — crucially for dutyholders — made geotagged photographic evidence of as-built construction part of compliance (the BREL report).
- Extensions must meet element U-values (walls 0.18, roofs 0.15, floors 0.18, windows 1.4 W/m²K or better via area-weighted/SAP trade-off routes) and the glazed area rule of thumb: openings ≤25% of extension floor area plus openings covered over.
- Thermal bridging is now designed, not defaulted: junction psi values, the photographic evidence schedule and the commissioning records are exactly the golden-thread items a PD must coordinate.
What the Building Safety Act requires for Part L on non-HRB domestic jobs
For new dwellings, compliance runs through SAP 10: the dwelling must beat its notional counterpart on primary energy, carbon emissions and fabric energy efficiency, meet minimum fabric standards, and be delivered with a BREL (Building Regulations England Part L) report signed off with photographic evidence of foundations/substructure, external wall construction, insulation continuity at junctions, airtightness details and services — geolocated or otherwise verifiable photos taken during construction.
For extensions — the small practice bread and butter — the standard route is elemental: new thermal elements at the limiting U-values, openings at ≤25% of the extension's floor area (plus any covered-over openings), with SAP-based area-weighted or whole-dwelling trade-offs available when the client wants more glass. That trade-off calculation is design evidence: commission it at Stage 3, not after the glazing order.
The photographic evidence regime is where the BSA dutyholder mindset lands hardest. Someone must plan which junctions get photographed before they are covered up. As Principal Designer (BSA), building the photo schedule into the specification — and holding the images in the project record — is precisely the 'plan, manage, monitor' duty in action.
Key design risks and evidence needed (RIBA Stages 2–4)
- Stage 2: set the fabric strategy and glazing ratio early; over-glazed extension concepts need the SAP trade-off flagged before the client falls in love with the sliders.
- Stage 3: preliminary SAP (new dwellings) or area-weighted U-value calc (over-glazed extensions); junction strategy — accredited construction details or calculated psi values; airtightness target set against the ventilation strategy (Part F).
- Stage 4: full U-value calculations to BS EN ISO 6946/13370, window schedule with certified Uw values, thermal bridging schedule, photographic evidence checklist written into the preliminaries, commissioning requirements for heating and hot water (Domestic Building Services Compliance Guide).
Standards and references you should be citing
- SAP 10.2 — the Standard Assessment Procedure underpinning new-dwelling compliance and trade-off calculations.
- BS EN ISO 6946 / BS EN ISO 13370 — U-value calculation methods for opaque elements and ground floors.
- BR 443 — conventions for U-value calculations (the document that stops optimistic assumptions).
- BS EN ISO 10211 / BRE IP 1/06 — thermal bridge modelling and psi value conventions.
- ATTMA TSL1 / BS EN ISO 9972 — airtightness testing of dwellings.
- PAS 2035 — where the project is grant-funded retrofit, the whole-house retrofit framework that sits alongside Part L.
Limiting fabric values for extensions (AD L Vol 1, 2021)
| Element | New element in an extension | Notes |
|---|---|---|
| External wall | 0.18 W/m²K | Thermal element U-value |
| Pitched roof (insulation at ceiling) | 0.15 W/m²K | 0.15 also for flat roofs |
| Floor | 0.18 W/m²K | Calculated to BS EN ISO 13370 with P/A ratio |
| Windows / rooflights | 1.4 W/m²K (or WER band B) | Rooflights assessed in the vertical plane |
| Doors | 1.4 W/m²K | Includes glazed doors |
| Glazed area | ≤25% of extension floor area + covered openings | Exceed via area-weighted or SAP trade-off |
Common pitfalls that catch out sole practitioners
- Nobody owns the photographic evidence schedule, so the BREL report stalls at completion with the insulation long since covered.
- Window U-values quoted as centre-pane instead of whole-window Uw — the schedule should demand certified whole-window values.
- Thermal bridging left to default y-values in SAP, wrecking the margin the fabric spec was supposed to deliver.
- Over-glazed extensions 'approved' informally at 35% glazing with no trade-off calculation on file — the file, not the conversation, is what defends you.
Frequently asked questions
What photographic evidence does Part L 2021 actually require?
For new dwellings, the BREL report process expects date- and location-referenced photographs of each plot showing foundations/substructure, wall construction and insulation, roof insulation, junction detailing, airtightness measures and building services, taken before covering up. For the Principal Designer, the practical task is writing the photo checklist into the contract documents and collecting the images into the golden-thread record.
Can a domestic extension have more than 25% glazing?
Yes, via compensating measures: an area-weighted U-value calculation showing the extension performs no worse than a notional compliant one, or a full SAP comparison of the existing dwelling plus extension against the dwelling plus notional extension. Commission the calculation at design stage and keep it — it is the compliance basis for the whole elevation.
Does Part L apply to a like-for-like window replacement?
Yes — replacement windows are controlled fittings and must achieve 1.4 W/m²K or WER band B, alongside the Part F trickle vent requirements. FENSA-type self-certification or Building Control notification applies either way.
Track this natively instead of in Word templates
PD Pathway gives small practices an automated Parts A–T relevant-requirements tracker, stage-by-stage Principal Designer checklists and one-click compliance documents — built strictly for non-HRB projects. First project free, then from £10 per project or £99/yr unlimited.