Fivex, a commercial property investment company, has completed three mixed-use developments that include 'build-to-rent' properties that they will 'build to keep, not sell'.

Fivex managing director Lesli Berger said for these projects to be viable, government policies needed to be simplified and updated to reflect the changing needs and demographics of Australians.

Transforming commercial property into 'mixed-use' properties, these rental properties combine residential, retail and commercial space.

Their third mixed-use development in Double Bay has been completed and the property now has ground floor retail, office space and two floors of residential flats.

The 12 flats in the building are fully leased and the office and retail space is almost fully occupied.

Mr Berger said that without policy and planning constraints, the company could provide around 1,000 new build-to-rent properties as rental stock.

If we are going to solve the real housing crisis, we need a change in attitude in order to streamline the approval process at local government level and change zoning plans

If we are going to address the real housing crisis, we need a change in attitude to streamline the approval process at the local government level, change zoning, and increase subsidies at the state and federal level, he said.

Mr Berger believes that Fivex's mixed-use rental properties are helping to solve the rental crisis.

Demand for rental properties is increasing. Mixed-use properties can meet that demand and expand housing options in areas traditionally dedicated to business and meet the changing needs of the community, he said.

Mr Berger said they had to overcome "huge challenges" when converting and expanding commercial properties to accommodate multi-use.

Fivex builds properties to rent rather than sell, which Mr Berger said has changed the way they build with sustainability and longevity in mind.

Our mixed-use projects demonstrate how residential, retail and commercial space can be seamlessly integrated to meet people's needs for an environment that combines productivity, leisure, culture and convenience in one place, he said.