Building onto my property

I have an extension onto the back of my house that is about 4 inches away from the split with my neighbour’s semi-d. The extension extends about 10ft out.

My neighbour is now building an extension also with the same dimensions. He asked me if it’d be ok to join his extension up to the wall on my extension instead of having a small gap. So effectively he’ll be building into 4inches * 10 foot strip of my property.

Should I allow this or what should I do? Should I charge him?

Well think if you where to sell in the future or him this could cause problems, The new buyer maybe could be in a position to ask them to take it down due to part of it been on their property, You could maybe sell the 4 inches for a small amount, with him having to look after the legal costs, this way in the future him or you selling up would not have this as an issue , even if his drains are on your land and not the building itself can cause problems, If it was me I would explain what could happen, also if u had a falling out over anything then you know what could happen if you do not go the legal way