Part M Low Threshold Doors Explained | Window Fitters

Part M Door Thresholds: Accessibility Requirements Explained

When planning a home extension or upgrading external doors, modern building codes place a strong emphasis on accessibility. Approved Document M (Access to and Use of Buildings) of the UK Building Regulations outlines strict requirements for door thresholds to ensure wheelchair users and individuals with reduced mobility can enter and exit buildings safely. Navigating these rules is essential when installing new entrance or patio doors.

Expert Reviewed by Marian Gheorghe, Head Window Fitter — Last reviewed: July 2026

What is a Door Threshold?

A door threshold (or sill) is the horizontal bottom member of a door frame that sits on the floor. In traditional UK building practices, thresholds were high (often 50mm to 75mm) to act as a barrier against wind and rain. However, these high steps create a significant trip hazard and present an impassable barrier for wheelchair users.

Approved Document M addresses this by mandating low thresholds (often called flush or level thresholds) for primary entrance doors and accessible garden doors.

The Key Rule: Maximum Step Height

To comply with Approved Document M, the threshold of an accessible entrance door must meet several strict dimensional criteria:

1. Maximum Total Height: The overall height of the threshold must not exceed 15mm.
2. Beveled Edges: Any raised edges on the threshold must be beveled or chamfered. Edges taller than 5mm must have a slope no steeper than 1:2.
3. Internal and External Floor Levels: The difference in height between the internal floor and the external paving should be minimized, and ideally, be flush.

These requirements ensure a wheelchair can roll smoothly over the threshold without getting stuck or tipping, and reduce trip risks for all occupants.

Part M Thresholds on Bifold and Patio Doors

Level thresholds are highly sought after for aluminium bifold doors and sliding patio doors, as they create a seamless transition between indoor living spaces and external decking or paving. However, achieving a flush threshold requires careful technical coordination:

• Weatherproofing Challenges: Because low thresholds sit close to the external floor level, they are more vulnerable to water ingress during heavy rain and wind. To prevent water from crossing the threshold, the installation must include an integrated drainage channel positioned immediately in front of the door sill to collect and divert water away from the structure.
• Sinking the Track: To achieve a flush finish where the internal floor and external decking are at the same level, the concrete subfloor must be prepared early to allow the aluminium track to be \"sunk\" below the finished floor level. Retrofitting a flush threshold into an existing opening without subfloor preparation is difficult and often results in a small internal step.

When Do Part M Rules Apply?

Accessibility rules apply based on the type of project:

• New Build Properties: Must comply fully with Document M. The primary entrance door and at least one set of garden doors must feature low thresholds.
• Building Extensions: If you are extending a home and creating a new opening, the local authority may require the door to meet accessibility guidelines, especially if the extension replaces the home's main accessible entrance.
• Like-for-Like Replacements: If you are replacing an existing door in an older private home, you are not legally required to upgrade to a low threshold, provided the new threshold is not higher than the old one. However, we advise clients to consider low thresholds for future-proofing their homes.

At Window Fitters Ltd, we have extensive experience installing Part M compliant doorsets. We coordinate frame measurements and subfloor preparation to ensure your installation meets all building codes while remaining weatherproof.

About the Author: Marian Gheorghe

Head Window Fitter & Active Director, Window Fitters Ltd

Marian Gheorghe is the Head Window Fitter and Director of Window Fitters Ltd. He has spent over 15 years installing high-security composite doors and aluminium bifold systems in London, specializing in subfloor leveling and weatherproof low-threshold integrations.

Connect on LinkedIn → Expert Reviewed: July 2026

Part M Threshold FAQs

Are low thresholds drafty?

No. Modern low thresholds feature double or triple-compression seals and magnetic gaskets that rise to seal the gap when the door closes, preventing drafts while keeping the threshold low.

What clear opening width is required by Document M?

Document M requires entrance doors to provide a minimum clear opening width of 775mm (when measured from the face of the frame to the edge of the door leaf when open at 90 degrees) to allow wheelchair access.

Can I install a low threshold door on timber decking?

Yes. Low threshold doors are popular for timber decking and paved patios. The decking boards can be installed flush with the door track, provided there is a drainage gap underneath to channel rainwater away.