Often not, if it is a house rather than a flat and the extension stays within permitted development. A single storey rear extension can go out 4 metres on a detached house and 3 metres on any other, up to 4 metres high, and anything within 2 metres of a boundary can have eaves no higher than 3 metres. A side extension has to be single storey, no more than 4 metres high and no wider than half the original house. Nothing can come forward of the front of the house, and extensions and outbuildings together can cover no more than half the land around it.
There is a larger allowance for single storey rear extensions, up to 8 metres on a detached house and 6 metres on others. It needs prior approval from the council, which asks the neighbours and judges the effect on them alone. A two storey extension under permitted development can go no more than 3 metres beyond the back of the house and has to stay 7 metres from the rear boundary.
Materials have to look similar to the house. In a conservation area the rules are tighter, with no side extensions, nothing over one storey at the rear and no cladding.
Much of Romney Marsh is in Flood Zone 3, which matters if the extension needs a planning application. The application then has to deal with flood risk, and the Environment Agency's standing advice asks for floor levels at least 600 millimetres above the estimated flood level, or flood resistance and resilience built in where the floor cannot go that high. An extension built under permitted development does not need this.
Every extension needs building regulations approval, whatever planning says, and a completion certificate at the end. If the foundations go close to a neighbour's, the Party Wall Act may need notice served on them first.
