Don't Move, Improve! is open to practices and homeowners that have carried out house extensions anywhere in London since 1 September 2016.
Organised by New London Architecture (NLA), an independent forum for issues surrounding London’s built environment, the competition encourages the "best design in everyday life and promotes the work of emerging practices".
Participants are judged on their innovation, liveability and cost-effectiveness. Whether the design "adds to London's character" is also taken into account.
"As the mayor's new draft London Plan calls for land intensification around the capital, these extension projects provide a showcase for the huge range of design-led options to better utilise space in the capital," said the organisers.
Entries will be judged by a selection of industry experts, including Dezeen editorial director Amy Frearson , NLA chairman Peter Murray, architect Carl Turner and journalist Philippa Stockley.
Last year's overall winner was a revamped Georgian townhouse with a patio that transforms into a reflecting pool.
Winning entries will be awarded a 12-month membership to the NLA, inclusion in a two-month public exhibition and magazine, as well as a series of consultation sessions to meet with potential clients.
Submissions are open until 18 October 2018 and winners will be announced at a ceremony in the new year.
For more information and to enter, visit the NLA website.