I agree wholeheartedly! The right hand side of Dene House is much earlier than the left which is why I said it may be an extension to an earlier building. It’s interesting to learn that the house had a stable. Could that have become the office block? That building, as can be seen from the photo has undergone several changes: The central, upstairs window has been removed and the two doors are a later addition as evidenced by the dividing upright and lintels, which seem to be much newer than the window lintels. It’s possible this has been an archway, wide enough for a carriage, leading to a small stable yard – possibly the walled area behind and to the side of the house with a single storey covered area on the side nearest the river, as shown in the first photo. It wasn’t uncommon to find married accommodation above the stables, which would explain the windows.
I might add, that the advert you mention states that there are 80 years left on the lease, suggesting that the house is about 20 years old in 1757 (Leasehold is generally 99 years).