23. Approval of documents for purposes of building regulations.

For the purpose of providing practical guidance with respect to the requirements of any provision of building regulations1 the appropriate national authority2 or a body designated3 by the appropriate national authority for these purposes may approve and issue any document4 (whether or not prepared by the appropriate national authority or by the body concerned), or approve any document issued or proposed to be issued otherwise than by the appropriate national authority or by the body concerned, if in the opinion of the appropriate national authority or, as the case may be, the body concerned the document is suitable for