what happens if a building is not designed to brick dimensions?

The only problems with not working full dimensions is that you will have cuts in brickwork i e three quarters which can be no less than 150ml , it’s nice if all works brickwork but not the end of the world if it doesn’t a good bricklayer will get around this

A good bricklayer can get past any issues with regard to dimensions. Ideally when a house is designed with facing brickwork then the architect should make the openings work to brick bond to look 100% right. There is no structural problem with non brick bond dimensions, it purely comes down to making it look good using the skills and knowledge of the bricklayer.

Correct, if the drawings are not to brick dimensions then you will have to cut 3 quarter bricks to get your bond, it won't affect your roof as long as the building is square.

The roof while be the main problem being out of square