For whatever it is worth...
I built an extension to our house, 1 storey, 400 sq feet ( 1 double bed, bathroom , living room - Independant electric/water/heating, e.g. entirely self contained with no service depencies to main house, so that added cost but flexibility as medium term plan is to rent the 3-bed house + rent a large studio, so that optimizes our revenue on the land)
I did everything myself from drawings/PP (application cost about €150 in total between ad in paper/maps from ordnance survey and fee to council) to finished product fully decorated. Only employed a contractor for plastering the outside walls, and skimming the inside.
Total cost : €19k.... and a lot of hard work week-ends/evenings for a year !!