A requirement for own door duplexes to have private open space of 10 sq m per bedroom will effectively kill duplex type development as a typology.
Typically, the duplex typology has evolved so that the larger own door duplex unit sits over a smaller ground floor own door apartment, with the private open space of the duplex unit being provided at 1st floor level in the form of a terrace. If the ground floor unit is a 2 bedroom unit with a footprint of say 80 sq m and the duplex unit above is a three bedroom unit with a footprint of say 60 sq m per floor, this only allows 20 sq m as a terrace, restricting the upper unit to a two bedroom unit.
The duplex typology is an attractive affordable alternative to traditional houses, which has been especially appealing to owner occupiers. Construction costs are on a par with house construction costs and maintenance costs are minimal. It’s a useful way of achieving the higher densities nowadays required in housing schemes without having to build apartments, and offers a broader range of options to the house buyer.
While some homeowners want a garden and therefore continue to opt for houses, not everyone does. For homeowners who want an ‘own door’ home, but have no interest in a garden, the own door apartment/duplex is an attractive offer.