Smelly Duvet

Hi, We are moving into a semi-detached house next to a neighbour who had their one storey side extension plan approved. The final plan is to keep a gap away from our wall instead of sharing a party wall (ours was an extension built by previous owner right up to the boundary). Although we do prefer not sharing a wall, the approved floor plan does not specify the width of the gap. Can we assume it to be 50mm as required by law? If so we are a bit concern about dampness and debris falling into it as it is not possible to be cleaned (and it’s on their side). Will we be better off to let them share our wall? With only 50mm left between us, will the visual “terracing effect” plus all the cons mentioned above lead to devaluation of our house? Appreciate if anyone can share your experience. Thank you!

Hi all, first time buyer here. Wish to learn from experienced builders here and DIY our new home. Happy to share our experience too!