5.—(1) The Board must assess the amount of levy to be paid in respect of each construction establishment of an employer in respect of the levy period.
(2) In this Order, “construction establishment” means any particular establishment of the employer engaged wholly or mainly in the construction industry for the necessary period.
(3) In this article “the necessary period” means—
(a)a period (which need not be continuous) consisting of a total of 27 or more weeks falling within the base period; or
(b)in the case of a construction establishment which started being engaged in the construction industry during the base period, a period (which need not be continuous)—
(i)falling within the base period; and
(ii)consisting of a total number of weeks exceeding one half of the number of weeks in the part of the base period beginning on the day on which the construction establishment started being engaged in that industry and ending on the last day of the base period.
(4) The person who on the first day of the levy period owns or otherwise has responsibility for a construction establishment is to be treated as the employer of all persons employed at or from that establishment during the base period.