I agree with Rob - this is a tricky subject. I did a bit of experimenting with the mosaic plugin that did a moderate job of giving the impression of houses. It discusses using shaped gradients to give you peaked roofs which will almost certainly help you even if you don't use the mosaic plugin to generate the shapes. Give me a shout if anything in there doesn't work as expected and I'll dig out my notes on it.

Using the Mosaic plugin to generate towns in Gimp