Clueless2023

Hi all, I am completely in the wilderness here, so any constructive advice would be much appreciated. i have a semi detached bungalow that I purchased in 2019. The previous owners had built a single storey side extension (based on size and construction I would assume under permitted development) in 2016. However, it appears they did not even notify the council of it, no sign off, certificate of lawfulness etc. and no records on anything on the councils historic planning/control site etc. We found this out halfway through purchasing house, although it was downplayed by the sellers and solicitors on both sides. Ours just stipulated an indemnity insurance etc. also we had limited options to live at time and really wanted the house, so it didnt bother us that much. Fast forward to now, we want to do a hip-gable conversion with a rear dormer, as far as I am aware this would all fall under PD and we have no constraints in our area and it would be under 50m3. My question is, if/when council find out about the existing single storey side extension during the process, what kind of possible implications can that have, in general, and in regards to the proposed loft development? And would it be beneficial to apply for regularisation of the existing extension beforehand if so? Any advice is much appreciated, thank you.