(a) a representation in writing is made to a planning authority by any person that unauthorised development may have been, is being or may be carried out, and it appears to the planning authority that the representation is not vexatious, frivolous or without substance or foundation, or
(b) it otherwise appears to the authority that unauthorised development may have been, is being or may be carried out,
the authority shall issue a warning letter to the owner, the occupier or any other person carrying out the development and may give a copy, at that time or thereafter, to any other person who in its opinion may be concerned with the matters to which the letter relates.
(2) Notwithstanding subsection (1), where the development in question is of a trivial or minor nature the planning authority may decide not to issue a warning letter.
(3) A planning authority shall issue the warning letter under subsection (1) as soon as may be but not later than 6 weeks after receipt of the representation under subsection (1).
(4) A warning letter shall refer to the land concerned and shall—
(a) state that it has come to the attention of the authority that unauthorised development may have been, is being or may be carried out,
(b) state that any person served with the letter may make submissions or observations in writing to the planning authority regarding the purported offence not later than four weeks from the date of the service of the warning letter,
(c) state that when a planning authority considers that unauthorised development has been, is being or may be carried out, an enforcement notice may be issued,
(d) state that officials of the planning authority may at all reasonable times enter on the land for the purposes of inspection,
(e) explain the possible penalties involved where there is an offence, and
(f) explain that any costs reasonably incurred by the planning authority in relation to enforcement proceedings may be recovered from a person on whom an enforcement notice is served or where court action is taken.