Technical Booklet R Access to and Use of Buildings·Page 27
Clear space to leading edge of door in lobbies with single leaf doors
The clear space dimension 'a' to the leading edge of a door in single leaf lobbies must be at least 300 mm. For every 100 mm increase in this dimension, the overall lobby length 'L' may be reduced by 100 mm but should not be less than L minus 600 mm.
For lobbies served by single leaf doors - the dimension "a" should be not less than 300 mm but for every 100 mm increase in this dimension (resulting in an increased lobby width) the overall length of the lobby "L" may be reduced by 100 mm but should not be less than L minus 600 mm.
Source — /Users/richardhill/Documents/planning-arch-project/data/documents/regional/Technical Booklet R - Access to and Use of Buildings.pdf