

You’d have to, like, add a whole other layer to the inside or outside
That is, actually what they do, by my understanding. If the house isn’t brick, then when you need to replace the siding they will actually put an entirely new layer of sheathing on over the outside, something like Zip R that has poly-iso foam insulation and acts as an air barrier. They then can put siding back on that fits the original look of the house, hopefully using architectural elements and details that were saved from teardown.
Another way is to go from the inside, and rip out the walls to the studs while saving trim pieces and put in new insulation and replace the horsehair plaster with drywall. Then you’ll be dealing with special ordering non-standard modern double glazed windows in weird sizes, because if you wanted to use the standard window sizes you can’t use your beautiful old growth mahogany trim pieces lovingly carved for your whacky leaky windows.
The attic is often not that bad to insulate because there should be relatively few cut-ins and blown in cellulose can go everywhere, but then you miss out on your perfect gothic “Wednesday’s room” unless you want to spend even more money trying to figure out how to get all of those turret towers and vaulting and weird rooflines into your envelope.
So, it’s possible, just prohibitively expensive
I’ve had this kind of situation in my life, I helped out my mother around the house when I was young and home from school, and I took care of my grandmother when she needed someone in the house. I’m now a stay-at-home mom so I can say I genuinely do get the appeal.
Here are some questions you should ask yourself though, and really consider your answers.
What are your life goals? If you knew you were going to die in a month, what would be the first things that came to mind that you would be sad you didn’t accomplish? Does your current path lead to these things?
What is your plan for when your dad’s current financial situation changes? If he loses his job or business, if he retires, do you have other means of making an independent income or other people in your life you could make similar arrangements with?
Do you have any of your own income, money, or savings tucked away? If something happened between the two of you, do you have options to get out?
What sort of preparation do you have for if something suddenly happened to your dad, like in the event of an accident or heart attack? Are you in his will? On the deed/lease? Do you have a joint bank account? Are you a beneficiary of his life insurance?
Do you want a romantic relationship, partnership, or kids? If so, what steps are you taking to make that happen, and how would that fit in to your current situation?
What sort of social or support network do you have? Do you have friends who would let you stay with them if you needed it? Do you have people in your life you can connect to and who will give you outside perspectives?
Domesticity can be alluring because you’re directly improving the lives of people you love, can make your own schedule, you’re not selling your soul to a corporation etc, but it’s extremely easy for the situation to go bad. There are so many ways people have been trapped, isolated, abused, or suddenly found themselves in changing circumstances that turn a good thing into a personal hell. Just the day to day of things can make a decade go by before you realize you never took that trip or learned that skill or made that thing.
Don’t just try to make your answers fit your current situation just because change is uncomfortable. If this lifestyle appeals to you there’s nothing wrong with that, but make sure you have your own contingencies.