If there is already a signed contract, then no, you do not have to agree: once there is a contract, you an enforce the terms of the contract as is and do not need to agree to any changes, new terms, etc. They have to go through with the sale without the additional occupancy.However, if there is no signed contract, they can insist on this as a condition of accepting your offer: a seller is not required to take any real estate offer made to them, even a full (or even above full) price one. Since they have disceretion as to whether to accept the offer or not, they can require you to agree to this. If you do, make sure all the terms are in writing, so they are understood by everyone and can be enforced.