Strata Subdividing a Duplex in NSW: Engineering and Survey Steps

Duplexes are the bread and butter of Sydney’s infill market, and nearly all of them end up strata titled so the two halves can be sold separately. The build itself is straightforward. What catches people out is the sequence: what has to be approved when, which certificates depend on which, and where the engineering sits in all of it.

Here’s the whole sequence as I’d walk a first-time developer through it. Ten steps, roughly in order, with the engineering and survey work called out at each one.

Step 1: check the site can be strata subdivided at all

A strata plan can only subdivide Torrens title land, and it subdivides a building, so there has to be one. Section 9 of the Strata Schemes Development Act 2015 covers what can be strata subdivided. On the planning side, the LEP has to permit dual occupancy in the zone, and you need to check whether the council’s minimum lot size for subdivision applies to strata (in many LEPs it doesn’t, which is exactly why strata is the default for duplexes on smaller blocks, but the LEP wording varies, so confirm it).

Engineering check at this stage: can the site drain? A duplex on a rear-sloping block with no drainage easement is a problem that title type won’t fix. Find the legal point of discharge before anything else.

Step 2: get the detail survey

Levels, features, kerb levels, neighbours’ floor and ridge levels, services. Every consultant works from it, and the surveyor who does it is often the one who’ll draft the strata plan later, which helps. Order it before the architect starts.

Step 3: decide the approval pathway and the subdivision timing

Two decisions here.

First, DA or CDC. Dual occupancies can go through as complying development under the Low Rise Housing Diversity Code in the Codes SEPP if the site and design fit every standard. If not, it’s a DA. That’s a planning call.

Second, when to apply for the subdivision. Common options: include “dual occupancy and strata subdivision” in the one DA, lodge a separate subdivision DA after the build, or, where the dual occupancy went through as complying development, pursue the subdivision through the complying development pathway as well. Including it in the original DA is the cleanest because council assesses the drainage, the services and the lot layout once, and the conditions for the subdivision certificate are set from the start. Ask your planner which suits, because some councils prefer one route.

Step 4: the DA (or CDC) engineering

For a duplex the package is normally a stormwater concept plan with preliminary OSD sizing, a site works and levels plan, a sediment control plan and driveway grades to AS 2890.1:2004. For a CDC it’s the full detailed design up front.

The strata decision shapes the drainage design from here on. Because the site will end up as two lots plus common property, one stormwater system serving both dwellings and one OSD tank on common property is the standard arrangement. That’s simpler than the two separate systems a Torrens subdivision would need, and councils generally expect it. Where the tank sits (under the shared driveway is common) is a decision the architect and I make together, because it has to be somewhere that ends up as common property on the strata plan and can be maintained without going through one owner’s courtyard. Our OSD design for duplexes is built around that.

Step 5: read the conditions of consent for the subdivision

When consent lands, the conditions will include a group headed something like “prior to the issue of a Subdivision Certificate”. Typically they cover: works-as-executed drainage plans, OSD certification, a positive covenant and restriction on use over the OSD, a Section 73 Compliance Certificate from Sydney Water, separate services to each lot, completion of the driveway and landscaping, and payment of contributions. List them on day one, because several take months to arrange.

Step 6: the CC engineering and the build

Detailed stormwater to AS/NZS 3500.3:2021, full OSD calculations to the council’s policy, retaining walls where the levels demand them, and the driveway long-section. On a duplex there’s a party wall and everything on either side of it has to be separable: two water services, two sewer connections (or one with a compliant junction), separate electrical and telecommunications lead-ins. The stormwater drainage design should show clearly which parts will be common property and which will be within each lot, because the surveyor uses that later.

A word on builders, since this is the step where the choice matters. A builder who has done duplexes and understands the subdivision conditions will build the services and the OSD to be certifiable, not just to work. If you’re still choosing, 13 Homes in Western Sydney build duplexes with the subdivision in mind and have civil engineers on the team, which shows in how the site drainage gets built.

Step 7: works-as-executed and OSD certification

Once the drainage and the tank are in, and before they’re covered, the surveyor picks up the as-built pipes, pits, invert levels and tank dimensions. I compare the works-as-executed plan against the approved design, check the orifice and storage are as designed, and issue the OSD certification most councils require. This is the document the positive covenant refers to, and it’s the one that gets forgotten until the certifier asks for it three days before the occupation certificate is due. Get the surveyor out before the backfill.

Step 8: Sydney Water Section 73

Creating new lots triggers Sydney Water’s Section 73 Compliance Certificate process. It’s applied for through a water servicing coordinator, and Sydney Water assess whether the new lots are adequately serviced and whether any works (an extension, a new connection, a sewer adjustment) are needed. For a duplex it’s usually straightforward, but the process has its own timeline and the certificate is a hard prerequisite for the subdivision certificate. Sydney Water’s developing hub explains the steps. Start it during construction, not after.

Step 9: the strata plan

Now the surveyor’s main job. The strata plan shows each lot, the common property, and the boundaries between them. Under the Registrar General’s Guidelines, structural boundaries are defined by permanent structure (the face of a wall, the edge of a slab, the edge of a roof), shown as thick lines, and things like fences and pavers don’t count. The structural boundaries page is short and worth reading if you want to know why the surveyor keeps asking whether a courtyard wall is masonry or timber.

The plan also comes with an administration sheet, unit entitlements (usually from a valuer), any by-laws, and a section 88B instrument if easements or covenants are being created with the plan. The OSD positive covenant and restriction on use normally go in here, over the common property. The engineer’s job at this step is to confirm with the surveyor that the OSD, the shared drainage and any shared retaining walls sit inside what’s being drawn as common property. If the tank ends up half inside lot 1, the covenant is a mess and council will send the plan back.

Step 10: subdivision certificate and registration

The subdivision certificate under Part 6 of the Environmental Planning and Assessment Act 1979 is issued by council (or a registered certifier, where council allows) once every condition is satisfied. The Planning Portal’s post-consent certificates page summarises it. With the certificate in hand the surveyor lodges the strata plan with NSW Land Registry Services, and once registered, two new folios exist, one per lot, plus the common property folio held by the owners corporation.

The occupation certificate is separate from all this. You can have an OC and be living in one side of the duplex while the strata registration is still going through. But you can’t sell the other side on its own title until the plan registers.

Where the time goes

If I look back at duplex jobs that ran late at the subdivision end, it was almost never the strata plan itself. It was:

• The Section 73 started after practical completion instead of during construction.
• The OSD tank backfilled before the works-as-executed survey, so the surveyor had to work from the builder’s photos and council wouldn’t accept it.
• The positive covenant wording bounced between the solicitor and council twice.
• A shared retaining wall that nobody had decided was common property.

All avoidable with a list at step 5 and a bit of nagging.

If you’d like the engineering coordinated with the surveyor and the planner from step 1 to step 10, that’s what our development engineering service does for duplex and small multi-dwelling projects across Sydney. One drainage system, one tank, one set of drawings the surveyor can draw the strata plan from.

The duplex is the easy part. The sequence is what gets you the second title. Follow it in order and the last step is just paperwork.