I get this question all the time, and it's REALLY difficult to answer. It's impossible to answer in your case - because we don't know the size or condition of the existing home. However, these points (below) tend to tell me what to do with a structure:
1) Are the "bones" (wood framing) of the home worth saving? Generally NO if they a) have extensive rot or b) were poorly/lightly built in the first place.
2) Is the home worth saving due to location, size, architectural features, etc?
3) Is the cost of the renovation cheap enough vs the after repair value (ARV)?
4) If you want to change (add on to) the property, can the existing home accept the addition?
A couple of examples:
1) A client wanted to lift and add another level to a single story coastal home. This home was in a flood zone, it was old, and lightly built, although in good condition. It was more affordable demolish and rebuild.
2) A client wanted to renovate an existing 2 family home that was a massive 1800's farmhouse. This home was built from very heavy duty timbers, had nice architectural detailing, and very high ceilings. The home also had a rough exterior including damaged and rotted siding. We kept the home, and repaired the exterior. The home's frame / bones were in good condition.
As a side note, I don't believe that a "tiny home" is an investment. These are trailers, not buildings. A small home can be nice and a good investment.
Another note: if the home is rotting above, and the foundation is damaged below, there's little left to save.