Must a house be washed seven times with water, one of them with earth, if a person wants to live in a house that was previously inhabited by a family that owned a dog?

The default state of things is purity, and the house does not need to be purified unless you are certain of impurity. If the impurity is on the ground or anything connected to it, such as what has been touched by dog waste, it is purified by pouring water over it until the impurity is removed. There is no requirement for a specific number of washes or for using earth [for purification].

Some scholars are of the view that the ground and what is connected to it can be purified by dryness, such as by the sun and wind, or by transformation [of the impurity]. This opinion was chosen by Sheikh al-Islam Ibn Taymiyyah, while the majority [of scholars] believe that washing is necessary. If the impurity is removed by the sun or wind, or if it transforms, then the ground is pure and it is permissible to pray on it. However, it is more cautious to follow the opinion of the majority (washing).