When you assign spacebar for firing and then start constructing something (turret for example) spacebar controls facing AND constructing at the same time.
This means instead of building something you just switch the direction.

Maybe I'm blind but i see no way to change the key settings for building.... these just seem to take a subset of the movement keys and add some other key which you can't change.

A possible workaround would be if i could assign keys to the mouse which seems not possible either....