How can I clean a house that used to have a dog in it?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

If you are certain that a given place in your house received contact with the saliva of a dog or its other excrements, then you need to wash that specific place seven times, making sure that the remains of the impurity are completely removed. But if you are not certain of anything, then you do not have to wash any particular place.

Normally things are pure unless there is a certainty that they have had some contact with impurity. A mere doubt that a given thing has become impure does not make it impure.

Allah knows best.