Commentary for Bava Metzia 8:9

חייב להעמיד לו בית – [for the days of his rental].

If one rented a house to his fellow and it fell down, the owner must build him a new house. If it was small he may not make it larger, and if it was large he may not make it smaller. If it was one house, he may not make it two; if it was two houses he may not make it one. He may not reduce or increase the number of windows, unless it is with both of their consent. If one rented a house to another person and the house fell down he must build him a new house that is basically the same as the previously rented house. He may neither add nor detract from the size of the house. The renter may have specifically rented a small house and may not want a larger house. If he rented a house with one room (1b) he may not split the house into two rooms or build a one room house if the former was two rooms. Nor may he increase or decrease the number of windows. Since in all of these cases a person might prefer the other option, the owner is obligated to build the same type of house that was previously rented. If, however, the tenant agrees to a change and the owner agrees as well, they may of course make any change they wish.

היה גדול לא יעשנו קטן וכו' – and he that showed him a house and said to him (i.e. the prospective renter): “a house like this one I will rent out to you,” but when he provides the house and says to him: “this house I am renting out to you,” and it falls, he is not liable to build it. But if he said to him an undefined/unspecified, he must provide him a place that is called a house, whether it is large or small.