Is your map tiled in squares? If so, just setting a tile where a house is placed on to unpassable is not the best approach. What if the house is smaller/bigger than the tile? If its smaller than a tile, the unit could avoid the house by driving/walking near the house, but ON that tile.

If the house is sized 2 times a tile, it covers the surrounding map tiles half, too. I hope you get my idea.

Obstacles should be observed as objects with position and size. Not as grids of the map. Otherwise you will get problems when objects don't have the size of your map grid.

I am not very good at pathfinding, but that was the first thing which appeared to me when I started giving AI to my units in TANX.
I decided to go a completely other way then. The units got sensors, which look at the near surroundings and if the sensors collide with objects, the units adjust their direction/speed.

It would be great to get some more information about you level design. This could help in giving tips on how you implement your pathfinding the best way.

Take a look at the demo of opensteer. I did not use the library, but I got the idea from there and coded my own for my needs (much simplier).

http://sourceforge.net/project/showf...group_id=77546

But as I already said, it all depends on your game. I don't know anything about your level design, just want to point you to some problems which could appear.