Standards, codes and sources

There is no single standard for interior dimensions, and any page that implies otherwise is selling something. What exists is a layered set of documents with very different force. Knowing which layer a number sits in tells you how much you can bend it.

Layer 1: codes, which are law where adopted#

These are enforceable. A building inspector can fail work that does not meet them, and insurance and resale can both turn on them.

International Residential Code (IRC), published by the International Code Council and adopted in most of the United States, usually with state amendments. It governs the dimensions that affect safety: minimum ceiling height (7 ft / 84 cm, section R305.1), hallway width (36 in, R311.6), stair rise and run (7.75 in maximum rise and 10 in minimum tread, R311.7), handrail height (34 to 38 in), guard heights, bathroom fixture clearances (21 in in front, 15 in to a toilet centreline, P2705.1) and hearth extensions (R1001.10). It sets minimums, not good practice.

ADA 2010 Standards for Accessible Design and ICC A117.1 Accessible and Usable Buildings and Facilities. The ADA governs public accommodations and commercial facilities, not private homes. Homes are covered by the Fair Housing Act's design requirements only for certain multi-family buildings. Nevertheless the ADA numbers are the best-documented accessibility dimensions in existence, and this site quotes them as best practice for homes while saying plainly that they are not a home requirement.

Approved Documents (England and Wales), particularly Part K for stairs and Part M for access. The UK stair rules differ meaningfully from the IRC: a maximum rise of 220 mm, a minimum going of 220 mm and a maximum pitch of 42 degrees for a private stair.

Layer 2: planning guidelines, which are best practice#

Not law, but the closest thing the trade has to consensus. They are where the comfortable numbers come from.

NKBA Kitchen and Bath Planning Guidelines with Access Standards, currently the 2022 edition, published by the National Kitchen and Bath Association. Thirty-one kitchen guidelines and twenty-seven bathroom guidelines, each with a recommended dimension and an accessibility variant. This is the source of the 42 in and 48 in work aisles, the 36 in of prep counter beside the sink, the 15 in landing beside a fridge, and the recommendation that a toilet centreline sit 18 in from a wall rather than the code's 15 in.

IES Lighting Handbook and IES RP-11, published by the Illuminating Engineering Society. These give recommended illuminance by task and space, which is where the foot-candle targets on this site come from.

ANSI/BIFMA and ANSI/HFES 100 for workstation and seating dimensions.

Layer 3: reference works, which explain the numbers#

Human Dimension and Interior Space, Panero and Zelnik. The standard anthropometric reference for interior work: body dimensions at percentiles, reach ranges, seated and standing eye heights, and the clearances that follow from them.

Architects' Data, Ernst Neufert. The European equivalent and the more comprehensive of the two for room and furniture planning.

The Measure of Man and Woman, Henry Dreyfuss Associates. The origin of much of the anthropometric data that later standards codified.

Time-Saver Standards for Interior Design and Space Planning. Broad, dated in places, still the most complete single volume on space planning conventions.

Layer 4: trade practice, which is convention#

Some very reliable numbers are written down nowhere authoritative. The 57 in art centre line is museum convention. The 16 to 18 in sofa-to-coffee-table gap is design practice. The 2 to 2.5 times fullness ratio for curtains is workroom practice. These are stated as trade practice on this site, never dressed up as standards.

Where the standards disagree with each other#

Dimension	IRC	NKBA	ADA / A117.1
Toilet centreline to wall	15 in min	18 in recommended	16 to 18 in required
Clear floor in front of a fixture	21 in min	30 in recommended	30 x 48 in clear floor space
Doorway clear width	32 in for egress doors	Follows access standard	32 in min at 90 degrees
Hallway / route	36 in	Not addressed	36 in, 42 in at some turns
Counter height	Not specified	36 in typical, 34 in accessible	34 in max for a seated user

Every page on this site names the layer it is quoting. If a figure appears without an attribution, that is an error and we would like to hear about it.