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 each levy period referred to in article 3(1).
(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 relevant base period; or
(b)in the case of a construction establishment which started being engaged in the construction industry during the relevant base period, a period (which need not be continuous)—
(i)falling within the relevant base period; and
(ii)consisting of a total number of weeks exceeding one half of the number of weeks in the part of the relevant base period starting on the day on which the construction establishment started being engaged in that industry and ending on the last day of the relevant base period.
(4) The person who on the first day of the relevant 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 relevant base period.