A person authorised by the local housing authority may at any reasonable time, on giving 24 hours' notice of his intention to the occupier, and to the owner if the owner is known, enter premises for the purpose of measuring the rooms of a dwelling in order to ascertain for the purposes of this Part the number of persons permitted to use the dwelling for sleeping.
An authorisation for the purposes of this section shall be in writing stating the particular purpose for which the entry is authorised.