Your backyard might be large enough to build a second dwelling; or the land could be wide enough to accommodate a duplex.
You will need a Planning Permit for the dual occ and a Building Permit before you can start to build.
You should allow cost for construction, subdivision costs and soft costs to cover consultants fees.
Building cost will depend on complexity of design, finishes, site constraints, single or double storey, and location of services to name a few factors which play a part in building cost. A Cost per square metre of $900 upwards plus subdivision costs of $10,000 upwards is a guide. That will cover a driveway and connection of services. Additional crossovers can cost around $3000.
Soft costs cover town planning design and documentation, town planning report and building permit drafting, land surveyor and building surveyor, civil and structural engineer and soil and energy reports are the least number of consultants involved. Fees can be upwards of $10,000- $15,000 plus. Other consultants may be required they cover vegetation (arborist) , CHMP, BAL to name a few. Council planning application and advertising be around $1800 and depends on cost of construction. A MPL Levy is charged if build cost is over $1.06 million and that threshold can increase.
You should allow 6-15 months for town planning approval, about 4 -8 weeks for Building permit and construction delivery will depend on access and other constraints.
A 6- 8 month build is generally in the ball park but could be higher
Duplex by AuArchitecture
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