Domestic use is defined in section 218 of the ‘Water Industry Act 1991’:
218. Meaning of “domestic purposes” in relation to water supply.
218.─(1) Subject to the following provisions of this section, in this Act references to domestic purposes, in relation to a supply of water to any premises or in relation to any cognate expression, are references to the drinking, washing, cooking, central heating and sanitary purposes for which water supplied to those premises may be used.
(2) Where the whole or any part of the premises are or are to be occupied as a house, those purposes shall be taken to include─
(a) the purposes of a profession carried on in that house or, where─
(i) that house and another part of the premises are occupied together; and
(ii) the house comprises the greater part of what is so occupied,
in that other part; and
(b) such purposes outside the house (including the washing of vehicles and the watering of gardens) as are connected with the occupation of the house and may be satisfied by a supply of water drawn from a tap inside the house and without the use of a hosepipe or similar apparatus.
(3) No such reference to domestic purposes shall be taken to include a reference─
(a) to the use of a bath having a capacity, measured to the centre line of overflow or in such other manner as may be prescribed, of more than two hundred and thirty litres;
(b) to the purposes of the business of a laundry; or
to any purpose of a business of preparing food or drink for consumption otherwise than on the premises.