Accessible design dimensions

Accessibility is where interior dimensions become law rather than preference, and where a mistake is paid for twice: once when the work is built and again when it is torn out. It is also where people work in the wrong order. Fittings first and route last is backwards.

The 36 in route comes before any fixture#

An accessible route is 36 in (915 mm) of clear width under the ADA, allowed to pinch to 32 in (815 mm) for no more than 24 in (610 mm), which is the depth of a door frame and the reason the two figures exist as a pair. Part M works to 900 mm (35.5 in) in dwellings and AS 1428.1 to a 1000 mm (39.5 in) path.

A manual wheelchair is 24 to 27 in (610 to 685 mm) across the rear wheels, so 36 in buys 4 in (100 mm) of clearance in total and assumes straight-line travel. It stops being enough the moment the route turns into a door in a side wall, which is the argument in accessible corridor and passing dimensions and the reason hallway widths that work recommends 42 to 48 in (107 to 122 cm).

The door is where the plan fails#

Clear width is measured from the face of the leaf, open at 90 degrees, to the opposite stop, not the nominal leaf size. A 1 3/4 in (44 mm) leaf plus its hinge offset removes 2 to 2.5 in (50 to 65 mm), so a 36 in (915 mm) leaf yields about 33.5 in (850 mm) and passes while a 34 in (865 mm) leaf yields 31.5 in (800 mm) and fails. Every size is worked through in door sizes, swings and the space they need.

Width is only half of it. A forward pull needs 18 in (455 mm) of clear wall on the latch side and 60 in (1525 mm) of depth. Hardware sits 34 to 48 in (865 to 1220 mm) up and must operate at 5 lbf (22 N) or less, which rules out every round knob. Accessible door widths, hardware and approach gives the clearance rectangle for each of the six approaches.

The 60 in circle, and what it may overlap#

The 60 in (1525 mm) circle is 19.6 sq ft (1.8 m2), about a sixth of a small bathroom. The alternative is a T-shaped space inside a 60 in square with 36 in (915 mm) arms, each extending 12 in (305 mm) past the intersection.

The most useful permission in the standard is that the 30 x 48 in (760 x 1220 mm) clear floor space at a fixture may overlap the turning space. The two are not additive, and that is what makes a small accessible bathroom buildable. See wheelchair turning and manoeuvring space, accessible bathroom dimensions and the bathroom layout checker.

15 to 48 in: where every height comes from#

Every mounting height follows from one diagram, the seated reach envelope. Above 48 in (1220 mm) and below 15 in (380 mm) nothing counts as usable, and reaching across an obstruction costs height quickly.

The obstructed rows reshape a room. See accessible reach ranges, the standing equivalents in reach zones and storage heights, and accessible kitchen dimensions.

What has to be decided before the wall closes#

Grab bars are the clearest case of a dimension settled during framing. A bar resists 250 lbf (1112 N) in any direction, and no plasterboard anchor will do that. It needs blocking, in place before the board, let alone the tile.

The geometry is fixed: 33 to 36 in (840 to 915 mm) to the top of the grip, a 1.5 in (38 mm) gap to the wall, a 1.25 to 2 in (32 to 51 mm) diameter, a side wall bar at least 42 in (1065 mm) long starting no more than 12 in (305 mm) from the rear wall, and a rear bar at least 36 in (915 mm). Positions are in grab bar heights and lengths, and what else is worth building in early is in ageing in place dimensions.

Level changes: thresholds first, ramps second#

A change of up to 1/4 in (6 mm) may be square, and between 1/4 and 1/2 in (6 to 13 mm) it must be bevelled at 1:2. Anything more is a ramp: 1:12 (8.3 per cent) at the steepest, no run climbing more than 30 in (760 mm), and 60 in (1525 mm) landings top and bottom.

The arithmetic is unkind to small level changes. One 7.5 in (190 mm) step at 1:12 needs 90 in (2.3 m) of slope plus a landing at each end, about 17 ft 6 in (5.3 m) for a step that could have been designed out. UK guidance eases to 1:15 and 1:20 as the going lengthens and AS 1428.1 fixes 1:14, so 1:12 is not acceptable everywhere. Runs and landings are in ramp slopes and landing sizes.

Which standard applies, and where they part company#

The standards agree within an inch or two on geometry and differ on scope, which is the difference that matters. The ADA is one mandatory standard for public premises, while Part M applies to dwellings in tiers, so a developer may build to the visitable tier and stop. The comparison is in accessible standards compared, and what is law rather than guidance is recorded in the standards and codes reference. Almost none of this is expensive at design stage: moving a bedroom wall 6 in to find the transfer space in accessible bedroom and closet dimensions costs nothing on paper and thousands once plastered.