We built a rear extension next to our neighbours with a 6 inch gap between then. Our extension extended out a metre more than the neighbours. The original fence was put back between the gardens being fixed to our extension wall so running alongside our extension for the bit the exceeds the neighbours. I thought that this worked better for stopping the neighbours tenants from attaching things to our wall etc. All was good until the neighbour accidently started a fire by their back door setting light our fence and intimately both extensions were fire damaged. We now have the builders back in repairing our fire damaged extension. The builders have said to start the boundary fence at the corner of our extension rather than alongside it. I prefer to have a fence alongside to to protect our wall and to maintain the boundary line. What is the correct way to deal with the fence? The deeds to not attribute ownership of the boundary to either property.