Is Your Block Actually Suitable for a Duplex?

Vanessa Harrison
Jan 20
3 min read

Buying land with the intention of building a duplex can look straightforward on paper. Two dwellings. One block. Strong resale or rental potential.

But in reality, many blocks that appear “perfect” for a duplex aren’t actually suitable once constraints are properly assessed.

Before you commit to a purchase - or commission design work - it’s critical to understand what really determines whether a block can support a duplex without unexpected cost, redesigns, or delays.

1. Zoning Is Only the Starting Point

Zoning tells you what may be permissible, not what is practical.

Even if duplexes are allowed in the zone, other controls and site conditions can significantly limit what can actually be built, including:

Minimum lot dimensions
Setbacks and building envelopes
Private open space requirements
Site coverage limits

A “yes” at zoning level does not equal a buildable duplex outcome.

2. Block Size & Shape Matter More Than People Think

Two blocks with the same area can produce very different results.

Key questions include:

Is the frontage wide enough to accommodate two dwellings?
Does the depth allow compliant layouts without pushing buildings too close to boundaries?
Are there irregular shapes or tapers that reduce usable building area?

Duplex designs are far less forgiving than single dwellings when it comes to geometry.

3. Slope, Contours & Earthworks Can Kill Feasibility

Slope is one of the most underestimated duplex risks.

Even moderate slopes can lead to:

Excessive cut and fill
Retaining walls
Split-level designs
Increased structural and drainage costs

In some cases, a sloping block may technically allow a duplex - but the construction cost outweighs the return, making the project commercially unviable.

4. Flooding & Drainage Constraints Are Often Overlooked

Flood controls don’t always show up clearly in marketing material.

Issues may include:

Flood planning levels
Overland flow paths
Stormwater discharge limitations
On-site detention requirements

A duplex doubles roof area, hardstand, and runoff - which can trigger more stringent drainage requirements than a single dwelling.

If these constraints are discovered late, redesigns are almost inevitable.

5. Easements, Services & Infrastructure Can Limit Layouts

Easements and underground services often restrict where buildings can sit.

Common examples include:

Stormwater or sewer easements along side or rear boundaries
Power infrastructure affecting setbacks
Existing pits, mains, or connection points limiting driveway placement

These constraints don’t always stop a duplex - but they can force compromised layouts or reduce achievable floor areas.

6. Access, Parking & Maneuvering Are Critical

Duplex sites must accommodate:

Separate access arrangements (or compliant shared access)
Minimum parking requirements
Vehicle maneuvering areas that comply with standards

On narrower blocks, achieving compliant access without sacrificing internal layout quality can be challenging.

7. “Buildable” Doesn’t Always Mean “Worth Building”

This is the distinction most buyers miss.

A block might:

Technically support a duplex, but
Require excessive retaining
Trigger costly drainage solutions
Result in compromised layouts
Reduce resale or rental appeal

The right question isn’t just “Can I build a duplex? ”It’s “Is this duplex worth building on this site?”

Why a Pre-Purchase Feasibility Matters

A pre-purchase site feasibility assesses constraints before you buy, including:

Zoning and planning controls
Flooding and drainage considerations
Slope and contour impacts
Physical site limitations
High-risk cost drivers

This allows you to make an informed decision - or renegotiate - before you’re financially committed.

Final Thought

Some blocks are marketed as “ideal for duplex” because they look good on paper.

Very few are assessed through the lens of engineering reality, risk, and cost.

Understanding that difference early can save you significant time, money, and stress.