19.—(1) A building warrant granted by a verifier shall, subject to paragraphs 2 to 4 below, be valid only for the period of three years beginning with the date on which that warrant is granted.
(2) The validity of such a warrant may be extended by a period of nine months where an application for that purpose has been made to, and granted by, a verifier before that period of three years has expired.
(3) Any period extended by virtue of paragraph (2) may be extended by a further period, or periods, if an application for that purpose has been made to, and granted by, a verifier before the expiry of the previous such period. Any such extension shall be for such period as that verifier may determine.
(4) Subject to paragraph (5), where the period during which a warrant is valid has expired, a verifier may nonetheless determine that that warrant should be valid for such further period as that verifier may determine.
(5) On approving a further period of validity of a building warrant on application in accordance with paragraphs (2) to (4) the verifier may, if the verifier sees fit, grant that approval subject to the condition that the compliance of any work or conversion carried out or made in accordance with the building warrant during the further period of validity shall be assessed against the building regulations(1) in force at the date of the application for a further period of validity.
(6) Where a building warrant relates to multiple subjects the verifier may require on application made to the verifier under paragraphs (2) to (5) that separate applications are made in respect of such of the multiple subjects as the verifier thinks fit.