The Workplace (Health, Safety and Welfare) Regulations 1992. Regulation 20, Sanitary conveniences, states:

1. Suitable and sufficient sanitary conveniences shall be provided at readily accessible places.
2. Without prejudice to the generality of paragraph (1), sanitary conveniences shall not be suitable unless –
   1. the rooms containing them are adequately ventilated and lit;
   2. they and the rooms containing them are kept in a clean and orderly condition; and
   3. separate rooms containing conveniences are provided for men and women except where and so far as each convenience is in a separate room the door of which is capable of being secured from inside.

The Approved code of practice goes on to give minimum numbers of facilities:

(The number of people at work shown in column 1 refers to the maximum number likely to be in the workplace at any one time)

Number of toilets and washbasins for mixed use