How would you design a house to survive a tsunami?

difficult, as you can have large steel boats slamming into the house. And you didn't specify the height of the tsunami. 20 feet? 2000 feet? And your question says you want the house to be tsunami proof, but later on you say only a shelter inside the house. The latter is a lot easier. Given the forces, the shelter needs to be underground to survive the forces that will bring any house down, unless it (the house) is made of foot thick steel with no windows. Given the underground shelter, it would be similar to a bomb shelter with foot thick concrete walls, floor and ceiling. The only additional feature is to make it water tight and add an oxygen supply. AND a way to let rescuers know that the shelter has living people in it under what could be a huge pile of rubble. The latter implies a large oxygen supply and some sort of underground telephone line connected to an exchange that is far away and at a higher altitude.

make it under the ground as the house will collapse