I am making a housewarming cake for a friend and would love to do a sheet cake with a 3d house on it. I was just wondering if I use like a 9x9 and stand it up right how do I dowel it so that it won't tip over?

Standing a cake on end is not a good idea. Rather, stack up layers. If you'd like the house to be narrow, just cut a sheet cake into thirds and stack the pieces with icing or filling in between.

Most likely, you'll want to place a board and dowels after the first two layers of cake for stability and ease of serving. You will definitely need a board and dowels in the base cake to support the house.

Finally, keep track of where you place your support dowels so that you can drive two long, sharpened dowels through the house, the base cake and into the base board for extra stability.

Wilton makes a standup house which might be what you need:
http://www.wilton.com/store/site/product.cfm?id=3E3191DF-475A-BAC0-53BA3BD42B6F8C98&fid=3E32B957-475A-BAC0-52A033E22009FA95

You would need to dowel underneath andplace the house on a small cake board. A dowel or two through the house and board and into the cake below would be safer, because the house is not 100% stable... it is a bit top heavy.

Thanks so much! Everyone is so helpful here. I just love it.

I would use the wilton pan it is easy and so cute and will help you get the look you are going for.