It's my understanding that if you own the property, and have the right blueprints anything is possible. I doubt the Counts and barons will be too keen on letting random players build their house too big, or too ugly and that may certainly be a major constraint on what can be done in any given area.

As for building on someone else's property, well if you have their permission I don't see why not. But it's been said that construction is a multi-person project and it will take time, so don't think you can do it behind the property owners back.

Sakaccino, I am pretty sure you can do all you said, but as JediAssassin said, in every situation you can have obstacles. But it is certainly possible mechanical.

Might be a good read for you.

https://chroniclesofelyria.com/blog/1203/DJ-14-Player-Housing-Architecture-Construction

It’s even possible to build bridges between two adjacent homes in order to combine them into larger structures.

1. See above :D only put this here cause markdown messed with the numbers on my list...
2. Yes, same as building over a road only simpler.
3. All buildings and blueprints can be modified and added on to, it just takes various degrees of skill to modify blueprints. Gotta know your stuff :P
4. Definitely doesn't have to be your property. How would contractors have a job if they could only work on their own homes, not to mention this would mean all homeowners have to take some time learning carpentry or some such.
5. I.... I don't see why not. That would be one heck of a structure though. I mean arguably underground settlements would fall into this category, no? From what I can tell, building wide seems easy, but building tall is what takes more skill.

If a settlement wall is considered a single structure, then it will be necessary for some structures to be larger than a single parcel.

I believe there will be some technical difficulty involved in making all that was mentioned happen. In all the other RPGs with construction that I played, there are deficiencies in the construction engine that prevent pieces from "snapping in place" if everything was not just so. They also mediocre at best at adjusting to terrain.