I'm always sceptical when someone like you asks a question, and there is no reply. I'd also like an answer to that, and also ask about planning permission, but I doubt I'll get a response.
@@alisonlee3314 3 years later and I have started the process. I have received planning permission for a 7x7 meter annex. My architect is now finalising building control to be submitted and getting drainage designed. After that I’ll approach 3 builders to quote.
This project was completed some 4 yrs ago the cost at that time unfurnished was some 40K. Of corse there was no land cost since it was built on existing property. Hope this helps.