Although it's correct to say 'Do you live in a flat or in a house?', it's not necessary to repeat the preposition when both nouns take the same preposition. Obviously if the preposition required for each noun is different then you would have to use both. For example. 'Do you live in a city or by the sea?'

In a flat + in a house = in a flat or house When you connect them with "and" or "or", you don't need to repeat the preposition. You don't need to repeat the article either! Compare: on a table + on a chair = on a table or (on a) chair. at home + at work = at home or (at) work.

both are the same, the house, the flat, the apartment , the loft. in a house, in an apartment, in a loft when the prepositions are the same you don't have to repeat but if you do, it is fine also.