In its most basic form, this is a set of template trays that have coral growing in them.
When the walls are thick enough, you stand them up and put the roof panels on top. The advantage of this is that your inside walls are flat. (as an option, the template could be transparent perspex that remains
attached to the wall)
The sides of the template should be angled so that the walls meet snuggly, maybe even dovetail. There are also 'islands' in the template for windows and a door.
Optionally, you could perfect the structure, but it's slightly more complicated. You submerge the assembled house, add polyps in the joins and let it glue the structure together (hehe, PolypFiller). During this time you could also add shelves, door arches, window sills, light fittings and any other shapes that could not happen in the original template.
If coral is not strong enough to support an ambitious design, beams, rods or a lattice or whatever can be placed in the template for the coral to go around, creating something similar to reinforced concrete.
Actually, you'd probably need to do all the wiring and plumbing at the template stage to avoid spoiling the finished texture of the walls.
Edit: these could also be placed in the sea to make a novelty diving environment.
Edit again: I think I'd also want to have wooden rafters that had barnacles grown on them.