An interesting thing I've just read is that
Polish houses often have a "half floor".
These are half-sunk levels with the main living area above. No doubt this has to do with the frost line being between 0.8m and 1.8m down in Poland (the frost line is the level you have to sink your foundations for us warm climate people).
This means your players are going to see Polish houses in a few new ways. This half level is a natural fighting position providing good cover and concealment. These half levels often have their own access, especially with the new builds.
There is standard methods of fortifying these sorts of structure. Usually the first thing to do is reinforce the floors above as much as possible so massive baulks of wood create a layer overhead combined with vertical heavy supports. This level is then sandbagged for a few layers.
Underneath the walls should be sandbagged as they are often only concrete block or double brick construction. After this blast walls are erected, these are baffles so that any munition penetrating the roof does not directly fragment the entire space. Behind every firing point should also be another sandbag wall to stop projectiles penetrating into the space and attacking defenders. As the firing points are at ground level they offer natural grazing fire for the area. Of course this is the optimum, you can even make these positions gas proof if you want go all out, but that sort of position is doable by a squad in a reasonable amount of time.
As Polish houses, especially in rural and regional areas, do not abut closely you cannot mousehole between them and covered trenches should be created between positions. These trenches should have traverses built in so they cannot be enfiladed. If a blast wall is placed in front of its entrance a loophole can be placed in it to cover the entry.
They are also a trap for vehicles. Trying to plough through a Polish house, never a good idea in the first place, should end up with the vehicle dropping a metre of so into the wreckage. You'll probably see unfortunate vehicles suffering this all over the place.
Of course, now that many buildings are abandoned many of these levels will be flooded.