Because of the word "in." There are a few prepositions (neban, an, vor, hinter, in, unter, über, auf, zwischen) that will cause either accusative OR dative, depending on the sentence. "Die Katze ist auf dem Tisch" (dative), vs. "Die Katze springt auf den Tisch" (accusative). What makes the second sentence accusative is the change of position of the cat (the cat was somewhere else first, and then it jumped onto the table). In the sentence we are dealing with here, "she lives in a house" is "sie wohnt in einem Haus' (dative). Someone please correct me if I'm wrong, but I believe that if she walked into a house (change of position), we would use the accusative. It takes a while to get the hang of this, and there are some trickier interpretations, but I believe this is generally how it works.
The ending of an adjective which follows an indefinite article (in this case, einem) and refers to a neutral noun (Haus) in the dative case is -en, not -es. http://german.about.com/library/weekly/aa033098.htm
Is still don't understand why it is grossen Haus. Isn't grossen an adjective (dative) for singular Haus? Isn't the -en ending for plural dative or masculine accusative?
When the noun has a determiner, or article - like dem, der, den, einem, einer, diesem, dieser... a, the, this, that, and so on - -en is the ending for all adjectives in the dative, regardless of number.
When the noun has no determiner, then the endings differ ('strong inflection'). This might help: