how do you make rectangular buildings non rectangular ?

The least I can do is to tidy up my own street.
How do I make non rectangular buildings, do I have to delete the original box and then draw a lline ?. if so, how do I delete the original ?, and is there a mode to make the sides parallel, cos it’s very tricky doing that by drawing a closed route. Most buildings have walls that are either paralllel or perpendicular so I guess this must be a common req. I live in a street that was build in stages in the early 1900s, almost every house is a different shape, and barely any are rectangular.

Far and away the best tool for doing fancy things with buildings are those in JOSM. Have a look at this page on the wiki

Shift-J in JOSM is very useful too as it can merge rectangular shapes to create more complicated ones.

Also the terracer plugin is neat.

Generally, you improve building shapes in the same way as you would improve ways: Add additional nodes into the existing way, then move the new nodes where you want them. Of course, it is hard to create perfectly straight lines manually.

Therefore, I suggest to experiment with JOSM’s “extrude” tool a bit. It is very helpful for drawing buildings that consist mostly of orthogonal lines, and is not limited to rectangles. Do so before you try any plugins. In my experience, the plugins only work well for very special cases, whereas the built-in tools are universally useful.

Watch this short video as an illustration. You can even combine this with manually dragging some nodes to create buildings where most, but not all, angles are orthogonal.