Extension built on neighbour's land

bris

They might not have done anything wrong, apart from take advantage of an elderly neighbor.
The OP states they got permission from the owner, and not all extensions need planning permission, so without knowing the full details, there may be nothing to complain about.

Extension to a House


In England Planning Permission is not required as long as:

• The proposed footprint does not exceed 50% of the total curtilage;
• The height would not exceed the highest part of the existing roof;
• The height of the eaves would not exceed the existing eaves height;
• The elevation does not front a highway;
• The maximum eaves height of the extension is less than 3m if it is within 2 metres of the boundary;
• The proposed extension does not exceed 4m in height or have a width greater than half the width of the original house:
• The proposed extension does not consist of - *the construction or provision of a veranda, balcony or raised platform;
  *the installation, alteration or replacement of microwave antenna;
  *the installtion, alteration or replacement of a chimney, flue or soil and vent pipe;
  *the alteration of any part of the roof of the original dwelling house;
• The materials used are similar to the existing house.