Millimetres decide whether a building works

Accessible design is a game of millimetres. A doorway that looks generous on plan can lose 60mm once you account for the frame and the door leaf. A turning circle that sits neatly between two walls can vanish the moment a radiator, a hand dryer or a door swing lands in the same spot. Once the partitions are up, fixing any of it is expensive, disruptive and occasionally impossible.

This is where computer-aided design earns its keep. Long before anyone sets foot on site, a well-built 2D drawing or 3D model lets you test clearances, walk the route, and see what happens when doors, furniture, services and people all occupy the same space at the same time.

Know the numbers, then put them in the model

UK guidance leaves less to interpretation than people assume. In England, Approved Document M covers dwellings (Volume 1) and other buildings (Volume 2); Scotland, Wales and Northern Ireland have their own equivalents. BS 8300 fills in the design detail. These are the figures you will return to constantly.

• Door clear opening width: 800mm minimum in most non-domestic situations, 850mm for dwelling entrance doors. Model the clear opening, not the structural opening — they are rarely the same.
• Turning circle: 1500mm diameter is the published figure. In practice, model 1800mm where you can, so feet and footplates have somewhere to go.
• Corridors: at least 1200mm for a single wheelchair user, 1800mm where two people are likely to pass — or a passing place at sensible intervals.
• WC and shower spaces: 750mm clear beside the pan, 1100mm in front, and roughly 900 x 1500mm of usable floor in a wheelchair-accessible shower room.
• Approach to a door: a 300mm nib to the leading edge on the pull side, and 1500mm of clear space in front of a door that opens towards you.

Always check the current tables — they are revised, and local interpretations vary. Having the numbers to hand simply makes the modelling decisions faster and less anxious.

Build accessibility into your CAD standards

The cheapest accessibility improvement you will ever make is a good template. Set up view templates, layer conventions and parametric components that carry their clearances with them, so nobody has to remember to draw a turning circle by hand at 5pm on a Friday.

• Door families or blocks with a parameter for clear opening width and a visibility state for the swing.
• Clearance zones as separate, switchable objects — turning circles, forward and side reach, manoeuvring spaces.
• Sanitary, kitchen and reception components that include the space a user needs, not just the object itself.
• A drawing-set convention where a single "access check" view shows routes, clearances and level changes together.

If you work in BIM, put accessibility data inside your components: whether a WC is ambulant or wheelchair-accessible, the gradient of a ramp, the clear opening of a door. Schedules and IFC exports then give you an audit trail rather than an argument.

Test the routes, not just the rooms

A compliant bathroom is no use if you cannot reach it. In the model, trace the accessible route from the site boundary, through the car park or footway, across the entrance threshold, along the corridor and into every key space. Overlay the clearance zones, switch on door swings, and place the furniture, counters and equipment that will actually be there.

Run interference or clash checks between door swings, clearance zones and services. Cut sections at 1:50 through the accessible WC, the reception counter and the lift lobby. Look at the 3D view at standing and seated eye level — it is remarkable how often a layout looks fine from above and hopeless from 1200mm.

Ramps, thresholds and the vertical bits

Level changes are where accessibility quietly fails. A 15mm chamfered threshold is fine; a 40mm one is not. Approved Document M limits ramp gradients and flight lengths — the steeper the gradient, the shorter the flight before a level landing is required. Model those landings, because they are the first thing to be value-engineered out of a scheme.

Check handrail positions, step nosings, the width of ambulant stairs, and lift car dimensions against the equipment you intend to specify. In a model, level changes are data as well as geometry, so you can schedule every threshold and gradient and check them in a table before you check them in concrete.

Checks to run before you issue the package

• Every door scheduled with its clear opening width, not its structural opening.
• Turning circles drawn at every change of direction, and unoccupied by furniture or radiators.
• An accessible route shown continuously from arrival point to destination.
• Section cuts through accessible WCs, ramps and lift lobbies.
• Gradients, thresholds and level changes listed and reviewed.

Finally, remember that a model is a tool, not a certificate. Get an access consultant, an access auditor or your building control body involved early. Bring them a model and the conversation becomes about solutions and sequencing rather than blame. That is a far better conversation to have in week six than in month twelve.