What is the maximum level difference at an accessible door threshold?

Part M permits a maximum upstand of 15mm at external door thresholds for accessible entrances. The upstand should be ramped on both sides at no steeper than 1:6 (a shallow bevel). Ideally, the threshold should be completely flush (0mm difference) with a drainage channel to manage water.

Why is a drainage channel needed at a level threshold?

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.

What type of grating should be used at a level threshold?

Gratings at level thresholds must be heel-safe with a maximum slot width of 13mm, orientated perpendicular to the direction of travel. 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.

How does a level threshold work with the DPC?

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.

Do all external doors need level access?

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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Level Threshold Calculator - Part M Level Access Door Threshold

Calculate level access threshold details for external doors to meet Part M wheelchair accessibility requirements.

Internal Floor Level (FFL)

Height of internal finished floor level above datum

External Ground Level

Height of external paving at door threshold position

Door Width

Overall door opening width (drainage extends 100mm beyond each jamb)

Door TypeType of external door at this threshold

Exposure LevelWind and rain exposure at this door location

Drainage TypeType of drainage channel at threshold

How We Calculate This

This calculator determines the threshold detail required to achieve level access at external doors per Approved Document M and BS 8300:2018.

Level difference

The calculator checks the difference between internal finished floor level (FFL) and external paving level. The maximum permitted upstand is 15mm, ramped at no steeper than 1:6 on both sides.

Drainage requirements

A drainage channel is typically required immediately outside the door line to intercept rainwater. The channel width depends on exposure level:

Sheltered: 100mm channel (or may be omitted with permanent canopy)
Moderate: 100mm channel minimum
Severe: 150mm channel with higher capacity

External paving fall

External paving must fall away from the threshold at a minimum gradient of 1:40 (2.5%) for at least 1200mm. This prevents water pooling at the door.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.