A lot of work, it has to be a labor of love. It will cost you twice what building new will, as well as present you with many, many PITA situations to work with/around.
I bought a 24’ x 28’, 2 story, balloon framed, abandoned 1856 farmhouse in NE PA. The walls on the first floor were so thick that the hardwood flooring, put in, in the 1930’s (we estimated by a car license plate under it) stopped 2.5 inches from the 1” thick Hemlock boards on the studs.
I did a 100% gutting, right down to stud walls. Jacked the main flooring beams up to near level (they were used barn timbers so that much older than the house!). New 200 amp service, new well pump, all new electric, plumbing, insulation….. Built my own kitchen cabinets out of an old horse barn I tore down (Mixed #3 hardwood) refinished the hardwood floors, put really nice sand-spackle designs on the ceilings around the single light in every room (Beautiful old timey look!). Bricked in two kitchen walls using recovered bricks and put a wood cookstove I refurbished there. … … It was going to be my forever home, so did everything up just the way I wanted it. You walked in, it looked and felt like an 1860’s farmhouse. I lived there while I was working on it. Thought I could save some money doing that and all the work myself. Ha… talk about learning a lesson the hard way!
Everything took twice as much time… and money... because you were having to work around what was there. You could not just put up a wall, you had to take down what was there before, patch things up, then put a wall in. You are cutting out and replacing a lot of timber. You also had to match it up to existing, which was never plumb or square. You simply cannot square up a house that old, you have to put in anything new with a certain ‘slant’ using existing lines or it looks really out of place.
You put in a new door, square in the frame and step back, it looks like it is wrong judging by everything else around it. You look at the gap between the top molding and the ceiling, there is 2” on one side and 3” on the other. The bottom of the door has ½” gap on the jam side and 1 ½ on the latch side. All the other lines in the room match up (as they have all settled together) but your door looks wrong. You have to work with it to make it fit in, so it functions and is visually appealing. Your new door is going to have to be shaved down to out of square, but still function and not be a ghost door.
Am I glad I did it? Yes, very much so! I had a lot of fun and really learned a lot about remodeling. I honed some valuable skills there that made me money over the years.
Would I do it again” No! NO friggin way! Someone could never pay me enough to tackle another job like that. I could build my own for a lot less and the end result would be almost the same… The new one everything would be square is the only difference!