We are thinking of doing the same, plus re-doing the roof (metal) because of the size of shed-dormer. We would need to put in a heat pump, floors, insulation, etc. We were quoted about $75K.

We are doing this exact finish of our attic OP. We've gotten quotes anywhere from $45-$75,000. There is some question about the stairs as they don't fit current code but two of the 3 contractors said that they'd be 'grandfathered' in. We are lucky that the floor is a lovely old pine that is in amazing shape so that may influence our pricing if you don't have a floor already and we have a slate roof so the new 'shed dormer' will cost a bit more even if they are able to re-use existing. What we are dealing with now is how much this work will effect rest of house. We may have to heavy up our electrical as we will need to add in a second HVAC system and to tie in the plumbing for attic bath we may have to mess with bathrooms below that we redid two years ago. Our house is 85yo and very well built but we wonder if we should just sell, move to another house as we are wondering about the can of worms we'll open once this work begins.

I looked into refinishing my attic and had exactly this problem. Joists could not support load if we made attic space living space. I would talk to a structural engineer first.

Your attic joists might not be big enough to be floor joists.

First make sure it's even possible. If it's an older home (as I'm guessing from lack of A/C), I'm guessing the attic/house in general was not built expecting the attic to be an additional floor of living space. You need a structural engineer to make sure the additional weight is feasible, especially if there's going to be a bathroom, walls, etc. You may need to add bearing columns or beams elsewhere. First get that info, then call a general contractor.

I think the location of your house and the actual sq footage has a lot to do with the cost.

We have a prettyl height attic that is not finished but is quite large, enough for study, bedroom, and bathroom. There is a stairway leading up to it already but we use for storage,. but I am guessing we would need to finish off everything, put in windows (maybe a shed dormer) , run plumbing , electrical, etc. We currently dont have a/c (we are in the northeast) but we were thinking of putting it in, so might want to coordinate that for ductwork.

Any idea how much we're talking about? If its under 100k we might do it.