My dp is quite bad at not doing things when he says he will.
I find the trick to motivate him is to start doing it myself. For example, if I wanted him to draw up plans, I would tell him I had booked an appointment with an architect and it was going to cost xx amount. I would make it clear that I was sorting it out. I don't know about your dh, but the thought of spending money where he didn't need to would be enough to motivate my dp.