Some examples listed below:
So-o, there has to be some building logic + freedom for the people to customize the size, shape and etc. Let's start from the bottom floor:
For any building you need a foundation under its walls, so the wall segments would be structurally stable + protected from erosion. Some medieval buildings had wooden foundations or none at all for timber housing, which lead to short building life. So, the player might have some options:
- build with a stone, rock or wooden foundation (will explain later why);
- build with no foundation at all and face repairing their buildings weekly for being a twat.
The foundation should take more stones/bricks than the pavement/flooring.

This is how the building floor is supposed to look. You may also do the wood flooring, that would require some more structural positioning. However, for the simplicity, it might be best leave it as it is shown in the pic.

Then you have your wall segments (wood-frame, stone) that you can build per each tile. You may build any opening only in the middle of 2 wall segments. Or, you may build a very narrow window in the middle of a segment. The windows or the doors shouldn't be too close to the corners.

You may also build columns outside or inside of the building. They have no sheltering purpose, only to structural. For instance, if your building is too long, you can place columns in the middle, so you can build the upper floors.

After your walls are finished, you may proceed to build the top floor. The top floor flooring should be built with wood. The floor can be overextended over any supporting structure for 1 tile.

Build your walls on top of it (for the overextended floors and wooden ground floor walls have to be wood, since rocks are too heavy):

Then you can proceed to build your roof with the wood and other materials used for the tiles (straw, turf, clay tiles, slate). Note, that this looks not so pretty because the building base area is quite small. It'd look better for a bigger scape buildings.


Other building configuration:


Some other things to mention, and general rules:
- you can build upper-level stone walls only if the foundation and lower levels are made of brick/stone (you could in real-life under some conditions);
- if the building is too long (10+ tiles) you have to build columns in the middle. Only then it will allow you to build the upper floor or a roof.
- if the building is built on shallow water it may have both wooden (columns) and stone foundations. If built on the deep water, then the wooden pillars only.
- Ideally, the devs should implement different types of walls segments. Some could be combined, others cannot (so it won't look ridiculous and will be believable);
- you may add openings (doors, windows and etc.) until the building is completed with a special button. So, you could customize the openings for the windmills, towers, fortifications and etc. and make them adjacent to each other;
- you can build only the timber structure over the 1 tile extention. It would be possible for the masonry only for 0.5 of a tile.
Overextended warts should have diagonal supports underneath:

To make the buildings look pretty, you could build the walls first, so the roof would aline around it:

Diagonal walls would be sweet as well:

Walls, openings, and etc. styles:
Everything listed above is the most obvious way of how the customizable building could work like. I will keep posting new topics with developed ideas, fortifications and etc. The major problem seems to be related to the roofs and making them allign to the walls.