Planning permission is not generally required to convert two attached dwellings into one larger property providing that the work is internal, and the buildings not listed. This is because no material change of use has occurred and internal alterations are not generally classed as development. If as part of the proposed works external alterations are also sought, then planning permission may be required depending on the scale and nature of the external works proposed.
It should be noted that if you do convert two attached dwellings into one larger property and then wish to convert them back to two dwellings in the future planning permission would be required.
If you require further advice on whether planning permission is required for your proposed works the Council offers a pre-application advice service.