Renovating a 19th-century mansion: should I keep the structure or start from scratch?

What really matters is doing a serious structural assessment before you decide. Get an engineer to properly inspect the foundation, beams, load-bearing walls, and check for moisture or pests. With that information you'll know if the structure can hold up or is completely shot. A 19th-century mansion usually has good bones if it hasn't been abandoned for decades, so often it's worth keeping it.

Preserving the structure has clear advantages: you spend less money, you keep the building's authenticity (which matters if you're going to sell it later), and those old mansions have details you can't easily recreate. Stone or old brick walls have their own character, even if you'll end up lining everything with modern insulation on the inside. What you're definitely going to have to change is pretty much everything else: plumbing, electrical, heating, ceilings, drainage systems. Yeah, that gets expensive.

I'd only recommend starting from scratch if the structure is really broken, there are serious differential settlements, or the structural renovation would cost about the same as tearing it down and rebuilding. But watch out: if it's a historically significant building or it's in a protected zone, you've got legal restrictions anyway. My advice is to get the technical assessment done first, talk to architects who have experience with renovations, and budget both options properly before you make a call.

Have you taken a close look at the walls and foundations yet, or are you still in the "I love it but have no idea where to start" phase? The thing nobody mentions is that often the structure is fine but the systems are totally outdated: lead pipes, electrical wiring that's honestly terrifying, no heating 😅 yeah, fixing that costs a fortune - almost as much as tearing it down and rebuilding - but at least you keep the character of the place, and honestly the demolition and permit costs will probably shock you with how expensive they are.