Dual Occupancy Feasibility: What to Check Before You Buy the Block

The purpose of a feasibility check is to produce a straight answer: this site supports two dwellings of roughly this size, at roughly this cost, over roughly this program. Anything vaguer than that is not feasibility, it is optimism with a spreadsheet attached.

The checks below cost very little and can all be done before you commit to a purchase. Skipping them is how investors end up owning a block that will not support the project they bought it for.

Start with what the planning scheme permits on the site. Each South East Queensland council has its own scheme — Brisbane City Plan, Gold Coast City Plan, the Logan and Ipswich schemes — and the provisions differ.

The relevant question is not only whether dual occupancy is permitted in the zone, but whether it is code assessable or impact assessable. Impact assessable proposals go to public notification, which adds time, cost and the risk of submissions.

In Ipswich there is an extra step: parts of the area, notably the Ripley Valley, sit in a Priority Development Area assessed by Economic Development Queensland rather than the council. Confirm which authority applies before anything else.

Frontage kills more duplex projects than lot area does. It determines the driveway, the parking layout, the setbacks, and whether two dwellings can be arranged on the block at all.

A site can comfortably satisfy the minimum lot size and still fail on frontage. Check it specifically rather than assuming that adequate area implies adequate width.

Find out where the sewer main runs. If it crosses the middle of the block, building over or near it requires approval and specific construction measures, and relocating a main is expensive enough to end a project.

This information is obtainable from the water authority before purchase. It is the single highest-value check on this list relative to what it costs.

Every site needs a legal point of stormwater discharge. Where the natural fall runs away from the street, the solution may require an inter-allotment drainage easement — which means negotiating with a neighbour who has no obligation to agree.

Check where the water goes before you assume it goes somewhere.

A geotechnical investigation establishes the site classification under AS 2870, which determines the footing system for both dwellings. Reactive clay is common across South East Queensland and an engineered footing system costs substantially more than a standard slab.

Uncontrolled fill from earlier subdivision is a specific risk in some growth suburbs. It has to be either removed or spanned, and both cost money.

Flood and storm tide overlays set minimum habitable floor levels, and on a duplex those levels apply to both dwellings. A high required level changes slab height, driveway grade and sometimes the whole structural approach.

Bushfire overlays trigger a BAL assessment with construction requirements attached. Both are visible on the council property report.

Two dwellings need more water and electrical capacity than one. In older suburbs the existing infrastructure sometimes cannot deliver it without an upgrade, and upgrade costs are borne by the developer, which is you.

Decide before design whether you intend to hold both, sell both, sell one, or live in one. The answer changes the design, the specification and sometimes the structure.

If the halves are to be sold separately, the survey plan and the subdivision or community titles path has to be planned from the start. Retrofitting a title split onto a design that was not built for one is expensive and occasionally impossible.

We run these checks before design on every dual occupancy project, and we will tell you plainly when a site does not stack up. That conversation costs nothing — our Brisbane duplex page and Logan duplex page set out how the numbers differ by area.