I don't think opacity will really work, but you have to try these things.

how about using a bit more colour, like tile colour. The rest of the map is nice and colourful, so as long as you don't go full-on pure red or anything it should compliment the greens and blues quite nicely.

EDIT: you could move the colour layer above all the houses layers and only have to do one?

EDIT 2: try varying the roof colour between buildings as well - just very tiny variations