Section 55 of the TCPA 1990 expressly provides that converting a single dwellinghouse to create two or more dwellinghouses will result in a material change of use requiring planning permission. However, the legislation is silent on whether combining dwellings (such as knocking two flats into one) would also constitute development.
The legislation excludes internal works from the meaning of development, however, combining residential units could still result in a material change of use. This was confirmed by the High Court case of Richmond-Upon-Thames London Borough Council v Secretary of State for Transport  2 P.L.R. 115, which held that where a change of use gave rise to planning considerations (such as the loss of residential accommodation), those considerations were relevant to determining whether or not the change was material. In that case, the conversion of seven flats to a single family house was a material change of use.
One of our clients recently applied to their local Council for a certificate of lawfulness, to confirm that the conversion of two existing houses into one house would not require planning permission. The Council issued the certificate, confirming that the conversion would not amount to a material change of use as there would be little impact on levels of occupancy, the character of the area or the type of housing available in the district. The Council also noted that it had no specific policy in relation to the retention of smaller units.
This result was as expected but, as with any case where the question is whether there has been a material change of use, each case will depend on its own facts. It is therefore always worth checking the position with the local planning authority and obtaining a certificate of lawfulness if planning permission is not required. As a starting point, it is sensible to check local policies for any restriction on the loss of particular types of dwellings.