Anniebling

Hi Everyone, Well I have googled and googled and ended up here, which seems to have a wealth of knowledge that goodness, I am looking for advice, 10 years ago we converted our detached double garage into an annexe 1 bed, living room, shower room and kitchen. We obtained change of use from the LPA to habitable room. I now want to split the plot and have the annexe as a separate dwelling. I want to extend the annex under permitted development with a side and rear extension. Then sell the original house and move into the annex. We have rented the annex for longer than four years so I think I can get it classified as a separate dwelling under the 4-year rule with a lawful development certificate. Sound like a plan? what are the pitfalls? Bound to be something to stop me. Any help or advice for a novice much appreciated