This is a general guide for homeowners who want to build a second dwelling on their RZ1 block that is larger than 90sqm. Second dwellings smaller than 90sqm are considered “secondary residences” and are subject to different rules.
This is general advice and is subject to a site inspection by our team.
Secondary dwellings and residences cannot be separately titled in RZ1 zones.
A dwelling is a building that includes 1 or 2 kitchens, at least 1 bath or shower and at least 1 toilet, and does not have access to another residential building.
There are two ways to build a second dwelling on RZ1 blocks: through a secondary residence or through a dual occupancy. A subset of dual occupancy is co-housing, where there are two separate dwellings with private living areas and some shared indoor components.
Both avenues will require Development Approval and Building Approval.
The block must be at least 800sqm to build a dual occupancy. If you want to build a second dwelling and your block is not over 800sqm, the block need only be over 500sqm to build a secondary residence, which is limited to 90sqm.
While a secondary residence must be 90sqm or less, there is no fixed numerical maximum to the size of a dual occupancy. Instead, the maximum size of a dual occupancy on an RZ1 block is determined by a maximum plot ratio (P). Plot ratio is the floor area of all buildings and structures on the block as a percentage of the total block area (B). For example, a 1000sqm block (B) with a 200sqm house and 50sqm garage has a plot ratio P of 250 ÷ 1000, or 25%.
The Multi Unit Housing Development Code establishes two maximums:
(1)The maximum permitted area of the new dual occupancy as a percentage of the total block B is:
( (140 ÷ B) + 0.15 ) ) ÷ 2
(2)The maximum permitted area of all buildings and structures (including new and old buildings) as a percentage of the total block B is:
( (140 ÷ B) + 0.15 ) )
The maximum allowable size of the block is the lesser of the two calculations. You can use the below calculation to determine the maximum permitted size of the dual occupancy:
Calculation 1: B x ( (140 ÷ B) + 0.15 ) ) ÷ 2
Calculation 2: B x ( (140 ÷ B) +0.15 ) - area of buildings (including garages, decks etc.) you intend to retain.
This floor area is indicative only and is the absolute upper limit. The actual size of the dual occupancy will be shaped by your design needs, utility easements and other Department rules (e.g. setbacks, building envelopes). Please meet with us to discuss this in greater detail.
Below is a table showing the largest permitted plot ratios. Remember to subtract retained structures from the “Maximum combined plot ratio”.
Maximum dual occupancy plot ratio
Maximum combined plot ratio
For example, on a 1486sqm block retaining 100sqm of original residence, the maximum using calculation 2 is 24.42% less 100 sqm, or 263sqm. However, the maximum using calculation 1 is 12.21%, or 181sqm. The maximum is thus 181sqm.
If you are demolishing an existing dwelling (e.g. demolishing a secondary) to build a dual occupancy, the dual occupancy must have at least 2 bedrooms if the demolished dwelling had 1 or 2 bedrooms, or at least 3 bedrooms if the demolished dwelling had 3 or more bedrooms.
Under Rule 38 of the Residential Zones Code, it is not possible to subdivide a dual occupancy built on an RZ1 block unless the dual occupancy was approved or constructed on or before 1 September 2003. It is possible to subdivide dual occupancies on blocks other than RZ1, subject to conditions.
Legislation specific to dual occupancies is found in the Territory Plan: the Residential Zones Development Code and the Multi Unit Housing Development Code. This is subordinate legislation made under s 46 of the Planning and Development Act 2007. Further legislation on building is found in the Planning and Development Regulation 2008, the Building Act 2004 and subordinate legislation, the National Construction Code and elsewhere.
This provides an overview of the basic rules for dual occupancy. The Territory Plan also imposes minimum requirements on issues including tree planting, offstreet parking and accessibility. Furthermore, utility companies with registered easements have specific requirements. These rules require a high degree of specificity. Please consult with us to discuss this further.