Summary
- All new dwellings must meet M4(1) 'visitable' as a minimum: level or ramped approach where reasonably practicable, an accessible threshold, WC provision at entrance level, and 750–900 mm clear door/corridor widths depending on approach angle.
- M4(2) (accessible & adaptable) and M4(3) (wheelchair) are optional requirements that bind only when imposed by planning condition — check the decision notice before designing, because M4(2) reshapes the plan (step-free approach, wider doors, future-proofed bathroom walls).
- Extensions must not make the existing dwelling less compliant — a rear extension that swallows the entrance-level WC is a Part M failure.
What the Building Safety Act requires for Part M on non-HRB domestic jobs
For new dwellings the baseline is M4(1): a reasonably level approach from the boundary or parking space, an accessible (nominally flush, ≤15 mm) principal entrance threshold, a WC at entrance level (or the first habitable storey where there is none), and internal door clear widths from Table 1 of AD M Volume 1 — 750 mm head-on, up to 900 mm where approached at an angle from a 900 mm corridor.
M4(2) and M4(3) only apply where a planning condition invokes them, but where they do, they are Building Regulations requirements in full: as Principal Designer (BSA) your first Stage 1 action on any new dwelling is to read the decision notice and record which category applies. Discovering an M4(2) condition at Stage 4 means redesigning stairs, bathrooms and door swings.
The accessible threshold is the detail where Part M collides with Parts C and L: flush access, damp-proofing and thermal continuity at the same junction. That threshold detail — drawn, not assumed — is a signature piece of PD coordination evidence on every new dwelling.
Key design risks and evidence needed (RIBA Stages 2–4)
- Stage 1–2: extract access requirements from the planning permission (M4(2)/M4(3) conditions); establish approach gradients from parking to door (1:20 preferred; steeper counts as ramped with its own rules).
- Stage 3: entrance-level WC located and sized; door schedule checked against clear-width table; corridor widths coordinated; for M4(2), check the bathroom wall zones for future grab rails and the stair for a future through-floor lift where required.
- Stage 4: accessible threshold detail (drainage, DPC, thermal break), door schedule with clear opening widths stated (structural opening minus frame and furniture), and switch/socket heights (450–1,200 mm zone) on the electrical layout.
Standards and references you should be citing
- Approved Document M Volume 1 (2015 edition incorporating 2016 amendments) — categories M4(1)–M4(3) and their space standards.
- BS 8300-2:2018 — design of an accessible and inclusive built environment (buildings); best-practice reference beyond AD M minimums.
- Accessible threshold guidance in AD M / 'Accessible thresholds in new housing' — the 15 mm maximum upstand detail.
- For commercial work, Approved Document M Volume 2 and BS 8300-2 govern — different document, same coordination logic.
Minimum clear door widths — M4(1) visitable dwellings
| Approach | Corridor width | Minimum clear door width |
|---|---|---|
| Straight-on | 900 mm | 750 mm |
| At an angle | 900 mm | 900 mm |
| At an angle | 1,050 mm | 775 mm |
| At an angle | 1,200 mm | 750 mm |
Common pitfalls that catch out sole practitioners
- M4(2) planning conditions unnoticed until Building Control plan check — read the decision notice at project setup and log the category in your requirements tracker.
- 'Flush threshold' written on the drawing with no detail resolving the DPC, drainage channel and cill — the words are not the compliance.
- Entrance-level WC door swinging over the clear activity space, or the WC removed entirely by an extension remodel.
- Clear width confused with leaf size: an 838 mm door leaf does not deliver 750 mm clear once frame, stops and projecting ironmongery are deducted — state clear opening widths on the schedule.
Frequently asked questions
Does Part M apply to a domestic extension?
Extensions to existing dwellings do not have to meet the new-dwelling access standards, but the work must not make the building less compliant than before — for example, removing the entrance-level WC or blocking the accessible entrance. Record the 'no worsening' check in your tracker.
How do I know if a new house must be M4(2) or M4(3)?
Only from the planning permission: these optional requirements apply solely where a planning condition imposes them. Check the decision notice at Stage 1, record the applicable category, and design to it from first layouts — retrofitting M4(2) into a finished plan is a redesign, not a tweak.
Track this natively instead of in Word templates
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