Let's figure this out .... Caveat ... I do not have insider real-time costs shared with me like buyer do ...
Random Example: 1725 Hidalgo Pass Ct, for sale is ~$213/TSF (Total SF, anything with a roof/foundation) & includes the lot, fees, etc.
The wet areas typically cost more than the other rooms, historically Bath = 103% of TSF
so we begin with $220 x your 215sf = $47,300 NEW
History strongly suggests that Remodel/Additions cost 150-200% of NEW and you want to avoid Low Bidders for such as this
Do we deduct lot price but then add small job & possible barriers to the work area or call this a wash?
Me thinks a wash.
Cost additions possible
Fixtures, cabinets, fenestration & finishes selected
Supply delays & out-of-stock
Design fees - e.g. impossible roofs marriage that could include demo costs, etc.
Walls demo
Risks
Addition foundations movement over time
Leaks
When you get an average of Bids later, please post the $$/SF, OK?
We are all still learning the post-/current COVID prices & supply chain.
Forgot to add other options:
A. You wrote "buying a new house" = house has a Builder who has the plans he can get your addition designed on to it, find you a new lot, then build ala NEW costs, predicatably lower
B. Builder gives you the plans to give to architect, then bid it out on a new lot
C. You take all these ideal plan ideas/concepts - free of any copyrights - to an architect to getRdone new on new lot
Ten words or less ... a goal unattainable