at one time MK used to do a single plate dimmer and switch combined wheras the two items one being a plain on off switch the other a knurled knob, which contolled the dimmer side, could be operated independantly. The other alternative is, again at one time there used to be available what I used to call a skeleton or extension pattress, which was the same as a normal plastic surface pattress but without a back in it. By mounting this onto the pd box it makes the whole thing that much deeper, and might allow you to use the grid system. If it was me I would carefully remove the old pd box, and again, carefully put a new deeper box in its place. you say new build most of the walls are normally stud or dot and dab so you might be able to use a dry lining box.