The opposition will push the government to scrap a 120-square-metre limit on secondary dwellings permitted across large areas of Canberra's suburbs under a new planning system. The Canberra Liberals say the size limit will prevent a wider range of housing sizes coming onto the market and will mean large blocks in older suburbs remain underutilised. Peter Cain, the party's spokesman on planning and land management, will on Wednesday move a motion in the Legislative Assembly calling for the government to scrap the restriction, which applies to second dwellings on RZ1 blocks larger than 800 square metres. The policy on dual occupancies was both not fit for purpose and an "unambitious" copy of the Canberra Liberals' own election commitment from the last campaign. "Why restrict a property owner on a quarter-acre block to building a 120 square metre second dwelling under only a unit title?" Mr Cain said in a statement. "It prevents a wider range of housing sizes being made available to the market. Local building and construction industry stakeholders are concerned that this size limit will act as a deterrent to construct a second dwelling. "Canberrans seeking to develop dual occupancies should not be restricted by such arbitrary and unproductive measures." Mr Cain's motion also calls on the government to allow RZ1 blocks to be subdivided into two separate residential leases, instead of only permitting a secondary dwelling under a unit titling arrangement on one residential lease. Chief Minister Andrew Barr and Planning Minister Mick Gentleman have previously defended the size limit, arguing 120 square metre homes are larger than people realise. Mr Barr told a Property Council lunch earlier this month a 120-square-metre home was "enormous" for two people. "I live in a 120-square-metre house, an extended ex-govvie in Dickson, and there's two of us and one very large cat," Mr Barr said. "We don't need that 250-square-metre house." Mr Gentleman told an annual reports inquiry earlier this month he had raised three children in a Calwell house about 120 square metres in size. "My three children, my wife and I all sustained ourselves in 120 square-metre three-bedroom home in Calwell. It's a very enjoyable house, and my kids have grown up well," Mr Gentleman said. "In my early years of growing up in Canberra ... we grew up in an 80-square-metre house, a two-bedroom FCC home in Reid. So I think it is an opportunity to grow up and have a family in that size residence." Changes to RZ1 zoning rules unveiled in September will permit second dwellings up to 120 square metres on blocks larger than 800 square metres, part of the largest shake up to Canberra's planning system since self government. Tree coverage and soft landscaping requirements will also apply to the blocks under the new system. An analysis published by The Canberra Times found 40 per cent of Canberra's RZ1 blocks would be eligible under the new policy, but many would not be suitably shaped for a secondary dwelling. Canberrans have the largest homes in the world, research released in February showed. Houses in the capital were more than double the size of homes in Wellington and more than five times the size of the average Moscow home. The research, released by UK company Buildworld, analysed property listing data to compare the average home size across 72 capital cities. Canberra topped the charts with the largest average home size at 256.3 square metres.