Summary
- Part C covers site preparation, contaminants (including radon) and resistance to moisture — the regulations behind DPCs, DPMs, tanking and sub-floor ventilation on every domestic extension and basement.
- Radon protection is location-driven: check the UKHSA/BGS radon map at Stage 2 and record the result, because a missing radon barrier cannot be retrofitted cheaply.
- Moisture failures are the most common domestic latent defect claim — your Stage 4 junction details and DPC/DPM continuity notes are the evidence that protects you.
What the Building Safety Act requires for Part C on non-HRB domestic jobs
Requirements C1 (site preparation and contaminants) and C2 (resistance to moisture) apply to all building work. On a typical extension the practical questions are: is the site in a radon-affected area, is there any history of contamination or filled ground, and are the floor, walls and roof detailed to resist rising, penetrating and interstitial moisture?
Under the BSA 2022 dutyholder regime the Principal Designer coordinates these answers. A desk study note at Stage 2 — radon map check, historic maps if the site history is doubtful, tree and drainage constraints — takes an hour and becomes the anchor of your Part C evidence. Waiting until Building Control asks is how projects stall at commencement.
Basements and retaining conversions raise the stakes: waterproofing design to BS 8102 should be led by a named waterproofing designer, and as PD you should record who holds that responsibility.
Key design risks and evidence needed (RIBA Stages 2–4)
- Stage 2: radon map check recorded with a screenshot or reference; site history note for contamination; ground conditions flagged for the structural engineer.
- Stage 3: DPC/DPM strategy coordinated with floor build-ups and thresholds (level access under Part M pushes DPC detailing hard); sub-floor ventilation route confirmed where suspended floors remain.
- Stage 4: junction details showing DPC/DPM continuity, cavity tray positions over openings, radon membrane and sump provision where required, and BS 8102 waterproofing grade for any below-ground space.
Standards and references you should be citing
- BS 8102:2022 — protection of below-ground structures against water ingress; specify the protection grade (Grade 1–3) explicitly.
- BR 211 (BRE, 'Radon: guidance on protective measures for new buildings', 2023 edition) — the standard reference for basic and full radon protection.
- BS 5250:2021 — management of moisture in buildings; the condensation-risk companion to your Part F ventilation strategy.
- BS 8215 — design and installation of damp-proof courses in masonry construction.
Radon and moisture decisions by project type
| Project type | Part C decision | Evidence to log |
|---|---|---|
| Ground-floor extension | Radon check; DPM/DPC continuity with existing | Radon map result, floor junction detail, cavity tray detail |
| Basement / cellar conversion | BS 8102 waterproofing grade and designer | Waterproofing design statement, named designer, maintenance note |
| Suspended timber floor retained | Sub-floor ventilation maintained | Vent calculation/positions marked on plan |
| Site with fill or past industrial use | Contamination desk study, possibly investigation | Desk study note, any specialist report |
Common pitfalls that catch out sole practitioners
- Extensions that block existing sub-floor air bricks without re-routing ventilation — a slow rot claim years later.
- Level thresholds detailed for Part M with no thought for driving rain, leaving the DPC bridged.
- Radon checked only at Building Control's request after the slab is designed.
- Basement conversions where nobody is named as the waterproofing designer, so BS 8102 responsibility falls between architect and contractor.
Frequently asked questions
How do I know if a domestic site needs radon protection?
Check the UKHSA/BGS indicative radon atlas for the postcode area. If the site is in an area where basic or full protection is indicated, follow BR 211: a radon-proof membrane across the footprint for basic protection, plus a sump or ventilated sub-floor for full protection. Record the map check either way — proving you checked is the point.
Who is responsible for waterproofing design on a domestic basement?
BS 8102 recommends a waterproofing specialist is included in the design team. As Principal Designer (BSA) you should ensure someone competent is named for that scope and record it — leaving it to the contractor's tanking sub-contractor without a design is a common source of uninsured failure.
Track this natively instead of in Word templates
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