Here are some tips that I have been told and learnt myself over time.
Anything you can't see, or are highly unlikely to see, should be removed. This is everything from the end of cylinders, bottom of boxes, and sides you will never see (hidden by another shape, or would only be viewed from an angle impossible to get in-game; eg: under the roof). Remember, if you have a 6 sided cylinder, there are going to be 6 polys on the top and bottom (assuming it is set to 1 cap segment), that you more than likely don't need (bottom almost definitely).
Don't do an interior; use textures to give the impression of. Leave details such as the bits around the edge of the roof, to textures with alpha channels.
Make cylinders/spheres/cones as low as possible without losing the effect. This depends on the size; the spires on the end and chimney pots could probably go as low as 5 or 6 sides. 8 would be the absolute max. The stanchions would be higher than the spies/chimneys, but still don't need to go high.
Window frames, door frames and other details should be done with textures.
Ensure that all boxes, planes and other shapes only have as many segments as required. For example, the planes forming the roof would only need 1 segment for length and width. The only time you need segments is when you are going to manipulate the shape somehow; eg: using a plane to make an arch.
Take a look at other peoples models. In some cases, a house is nothing more than a 4 sided box (top and bottom removed) with two planes on top for the roof. The rest is done using textures.