It is against the law to allow the spread of five specific types of weeds under the Weeds Act 1959 and 46 other weeds under the Wildlife and Countryside Act 1981 into neighbouring land.
In a new home, it is a good idea to build a root barrier 450mm deep at the boundary using paving slabs on-end set in concrete, to prevent neighbouring weeds and plant roots affecting your garden.
If the slabs are left projecting 100mm above ground it will also provide a splash zone to protect your new fences against rot and loading from soil being piled up against the fence by a neighbour.