Proposed larger home extensions
It is a condition of Schedule 2 Part 1 Class A of the General Permitted Development Order 1995 (as amended) [external link] that larger house extensions which are deemed to be permitted development must be completed by 30 May 2019, even where a determination has been issued by the Council prior to this date.
If the extension is not completed by then, planning permission would be required. This temporary arrangement expires on 30 May 2019, after which the Government will review the situation.
At the present time there is no indication as to whether this arrangement is to be extended beyond this date and so if you are unable to complete the development by 30 May 2019 then a full planning application should be submitted.