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Level Threshold Calculator - Part M Level Access Door Threshold
Calculate level access threshold details for external doors to meet Part M wheelchair accessibility requirements.
Height of internal finished floor level above datum
Height of external paving at door threshold position
Overall door opening width (drainage extends 100mm beyond each jamb)
A canopy may reduce drainage requirements in sheltered locations
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How We Calculate This
This calculator determines the threshold detail required to achieve level access at external doors per Approved Document M and BS 8300-1:2018.
Level difference
The calculator checks the difference between internal finished floor level (FFL) and external paving level. The maximum total threshold height is 15mm. Any upstand higher than 5mm must be chamfered or rounded — the LABC Warranty checklist gives a chamfer slope of between 7° and 15°.
Drainage requirements
A drainage channel is typically required between the landing and the threshold to intercept rainwater. The channel body width depends on exposure level (slot opening is separate — see below):
- Sheltered / Moderate: 100mm body width (may be omitted in a sheltered location with a permanent canopy)
- Severe: 150mm body width for higher capacity
The heel-safe grating slot opening must be no wider than 18mm (LABC Warranty checklist), orientated perpendicular to the direction of travel. This is the grating opening, not the overall channel body width.
External landing fall
The external landing must fall away from the door at between 1:40 and 1:60 (1:40 is the maximum/steepest, 1:60 the minimum) in a single direction, with no cross-fall at the threshold landing. Approved Document M defines ‘level’ as a gradient not exceeding 1:60. This sheds water away from the door while keeping the landing flat enough for wheelchair use.
Frequently Asked Questions
Approved Document M (Appendix A) permits a total threshold height of no more than 15mm at external door thresholds for accessible entrances. Any upstand higher than 5mm must be chamfered or rounded — the LABC Warranty Level Thresholds Checklist gives a chamfer slope of between 7° and 15°. Ideally, the threshold should be completely flush (0mm difference) with a drainage channel to manage water.
When the external ground level is close to the internal floor level (as required for wheelchair access), there is a high risk of rainwater entering the building. A drainage channel immediately outside the door line intercepts water before it reaches the threshold. This is essential for weather protection while maintaining level access.
Gratings at level thresholds must be heel-safe, orientated perpendicular to the direction of travel. The LABC Warranty Level Thresholds Checklist limits the grating slot opening to no wider than 18mm; many specifiers use a more conservative 13mm or less for heel safety. The grating must be firm, flush with the surrounding paving, and capable of supporting wheelchair loads (B125 class minimum). Avoid round-hole gratings as wheelchair castors can get trapped.
Maintaining the damp-proof course (DPC) integrity at a level threshold requires careful detailing. Specialist level threshold systems incorporate a cavity tray and drainage channel that sits below the DPC level while keeping the threshold flush. Always follow the threshold manufacturer installation guide and Building Regulations Part C requirements.
Part M requires level access at the principal (main) entrance to all new dwellings. For non-domestic buildings, all entrances used by the public should have level or ramped access. Other doors may have a small step, but at least one accessible entrance must be provided. Extensions and alterations should maintain or improve accessibility.
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Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.