Building regulations reference

Part L U-values (W/m²K)

Approved Document L Volume 1, 2021 edition incorporating 2023 amendments — Table 1.1 (notional dwelling), Table 4.1 (limiting values, new dwellings), Table 4.2 (new elements in existing dwellings), Table 4.3 (retained elements); Approved Document L 2026 for the Future Homes Standard notional dwelling. Checked 4 September 2026.

Every λ we calculate with — 418 materials and named products with their sources — is in the thermal conductivity library. Existing dwellings work the other way round: where the construction cannot be evidenced, RdSAP assigns a default from the age band — all of Tables 1, 6–10 and 18 are on the RdSAP age bands and default U-values page, with a year-to-band lookup. The notional dwelling sets the target a design is measured against; the limiting values are the worst any element may be. The Future Homes Standard notional keeps the 2021 fabric and tightens air permeability to 4.0; the Home Energy Model version differs only in how the notional heat pump is modelled — see the Future Homes Standard guide. Explained: U-values · Extensions: the 25% glazing checker.

Part L editions and transitional dates

The Building Regulations etc. (Amendment) (England) Regulations 2021 and 2026; Future Homes and Buildings Standards Building Circular 01/2026; Welsh and Scottish government publications. Checked 4 September 2026.

Transitional protection is per building, not per site. Not sure which edition your scheme falls under? Run the Part L edition checker. Explained: Part L · Future Homes Standard.

SAP: the carbon and primary energy factors, and the notional dwelling

SAP 10.2 Table 12 and BRE technical paper S10TP-15 (Part L 2021 work); SAP 10.3 specification (12-06-2026 version), Table 12 and Appendix R Table R1 (the Future Homes Standard); Notice of Approval, 24 March 2026 — SAP 10.3 is approved from that date, and buildings notified before 24 March 2027 and started before 24 March 2028 may continue in the earlier version; Approved Document L Volume 1 2021, Table 1.1 for the Part L 2021 notional dwelling. Checked 4 September 2026.

Fuel factors (Table 12)

The notional dwelling the targets come from

Not the same numbers: the annual DESNZ/Defra reporting factors used for energy statements and SECR are on the carbon conversion factors page — SAP's are fixed per version, those change every year. Part L 2021 work is assessed in SAP 10.2. SAP 10.3, approved on 24 March 2026, is the Future Homes Standard methodology: its electricity factors move both ways — carbon down to 0.086, primary energy up to 1.969 — and it uses the annual figures directly rather than monthly ones; what changed in SAP 10.3. The four tests a new dwelling passes at once — DPER ≤ TPER, DER ≤ TER, DFEE ≤ TFEE and the limiting values — are explained in Part L, DER and TER and TFEE and DFEE. SAP 10 changes: what's new.

Junction Ψ-values (W/m·K) — SAP's default and the notional dwelling

SAP 10.3 Appendix K, Table K1 — the default Ψ a junction gets when no value has been calculated — and Appendix R, Table R2, what the notional (target) dwelling assumes. Both are SAP's own published figures. We deliberately do not publish a calculated Ψ per junction here. A Ψ-value belongs to the detail as drawn: change the block density, the insulation thickness, the lintel or where the floor insulation sits and the number moves, sometimes a long way. A figure copied off a reference table is not a calculation of your junction, and Building Control will treat it as one. Solve yours instead — every row opens ΨMonkey with that junction loaded. Checked 4 September 2026.

Ψ in W/(m·K). SAP default is the Table K1 figure a junction gets when nothing is calculated (with no junction information at all, SAP uses y = 0.2 W/m²K across the whole envelope); notional is what the target dwelling assumes (Table R2). Both are SAP's own published figures. The gap between them is what calculating that junction is worth per metre — and the only way to claim it is a Ψ-value modelled on your detail, because the same junction reference moves a long way between a full-fill cavity, a timber frame and a service void. Solve yours free in ΨMonkey.

The complete set — every E, P and R reference with its Table K1 default, the Table R2 notional and the gap between them — is on the SAP junction reference. To see how far the same junction moves between a full-fill cavity, a timber frame and a service void, the Ψ-value library shows all 38 templates solved against eight named walls — worked examples to start from, not values to lift. Explained: what a psi value is · thermal bridging · ACDs and Recognised Construction Details · Whole-dwelling total: y-value calculator.

Air tightness (Part L)

Approved Document L Volume 1 2021, Regulation 43 and Table 4.1; Approved Document L 2026; Volume 2 for non-domestic. Checked 4 September 2026.

If the design SAP assumed 5 and the test comes back at 7, the as-built SAP uses 7 and the shortfall must be made up elsewhere. Service: air tightness testing, with the pre-test checklist.

Part O overheating — the simplified method tables

Approved Document O, 2021 edition (in force 15 June 2022): Tables 1.1–1.4, paragraphs 1.3–1.9 and 3.2–3.3. Glazing is the transparent area, frames excluded; limits are read for the façade with the largest glazed area. High-risk locations are the London postcode districts in Appendix C; everywhere else is moderate risk. CIBSE TM59 (2017) for the dynamic-modelling criteria and weather file. Checked 4 September 2026.

Maximum glazing area — cross-ventilated (Table 1.1)

Maximum glazing area — not cross-ventilated (Table 1.2)

Minimum free area for removing heat (Tables 1.3 and 1.4)

The other Part O numbers

Explained: Part O · Service: Part O assessments · TM59:2026 — what changed.

Part G water efficiency

Building Regulation 36 and Approved Document G; The Water Efficiency Calculator for New Dwellings (DCLG, 2009), Table 1 — the same constants WaterMonkey uses. Checked 4 September 2026.

Table 1 use factors

Consumption = Σ(capacity × use factor + fixed use) × 0.91 normalisation, plus 5 l/p/d external use. Is your site on 125 or 110? The optional requirement is engaged by a condition on the planning permission, not by a Local Plan policy — how to check, with the Environment Agency's water stressed areas classification. Why London's 105-excluding-external is the same target as 110: explained. Machine-readable: CSV and JSON. Service: Part G water calculations. Explained: the Part G calculation · 125 vs 110.

Part E sound insulation

Approved Document E, Table 0.1a (dwellings) and 0.1b (rooms for residential purposes); Regulation 41 pre-completion testing. Checked 4 September 2026.

Higher is better for airborne, lower for impact. Pre-completion testing is the default route; registered Robust Details are the alternative for new-build houses and flats. Service: sound testing.

Soakaways — BRE Digest 365 in numbers

BRE Digest 365 Soakaway design (2016 edition); Approved Document H. Checked 4 September 2026.

Not the septic-tank drainage-field test: that one uses a 300 mm square hole and gives a Vp in seconds per millimetre (a drainage field needs Vp 12–100) — the difference explained. Explained: BRE Digest 365 — the soakage test and the design method · the calculation row by row.

Nutrient neutrality — the 27 Natural England catchments

Natural England's nutrient neutrality advice and catchment calculators (NECR459 methodology, 20% precautionary buffer). N = nitrogen, P = phosphorus. Checked 4 September 2026.

Each catchment page carries the credit position where one is known. Permit consents: the wastewater treatment works permit register — 675 works across 22 catchments, nitrogen and phosphorus, current, post-2025 and post-2030. Where is my site? The catchment map. Credits and cost: nutrient credits calculator · Service: nutrient neutrality assessments. Machine-readable lists: CSV and JSON. Policy: the Nature Restoration Fund.

Regs calendar

Dates from the instruments, circulars and consultations linked in each row, as covered in our regs watch. Checked 4 September 2026.

Updates land in the news section and the RSS feed as they happen.