Granny flat: CDC or DA?
The fork that decides your timeline.

Secondary dwelling is the planning term. Granny flat is what everyone actually says. Either way it means a self-contained dwelling within, attached to, or separate from the main house on the same lot — and under the State Environmental Planning Policy (Housing) 2021 there are two ways to get one approved.

They are not interchangeable, and the difference between them is measured in months.

The two pathways

Complying development

A Complying Development Certificate rolls planning approval and construction approval into one. No council merit assessment, no neighbour notification, dramatically shorter. The catch is that every standard has to be met — there is no discretion available, and no partial credit. Miss one and you're not on this pathway at all.

Development application

Council assesses the proposal on its merits. Slower and more expensive, but it's the route for lots that don't meet the complying development thresholds, for constrained sites, and for designs that step outside the code for a defensible reason. And once the DA is approved you still need a Construction Certificate before you can build.

The threshold that decides it

For the complying development pathway your lot generally needs to be at least 450m², and the property has to sit in a zone where secondary dwellings are permitted — typically the residential zones. Below 450m², the CDC route closes. A DA is still available.

That's the first thing to check, and it takes about two minutes with a title search. It's also the check most people skip before commissioning a design.

The floor area rule that catches people

This is the one worth reading twice.

Most people know the number 60m². What fewer people know is that the way floor area is calculated for the complying development pathway was amended, and it now takes in the principal dwelling, the secondary dwelling, and any attached carport, garage, balcony, deck, patio, pergola, terrace or verandah.

Read that list again, because it's longer than anyone expects. Designs drawn to the older understanding — where the granny flat was measured more or less on its own — can fail on a calculation that has nothing to do with the granny flat itself. An existing double garage and a generous rear deck on the main house can consume the allowance before your new building is even on the page.

The most expensive granny flat mistake isn't a design problem. It's designing to a rule that changed.

Three more things worth knowing early

1. One principal, one secondary dwelling per lot. If you're picturing two granny flats, or a granny flat alongside a dual occupancy, that's a different application entirely.
2. You can't subdivide it off. A lot with a secondary dwelling on it cannot then be subdivided to sell the granny flat separately. If separate titles are the real goal, a secondary dwelling is the wrong instrument — you want a dual occupancy with subdivision, and it's much better to know that before you build.
3. The state policy is a floor, not a ceiling. Northern Beaches Council's own local environmental plan and development control plan can apply controls on top of the state standards. Satisfying one and not the other is a common and avoidable way to get knocked back.

What actually takes the time

Not the certificate. On either pathway, the long pole is documentation — BASIX, structural engineering, stormwater, Sydney Water, and whatever site-specific reports your land requires. A project arriving with a survey and a design already in hand moves quickly. A project starting from a bare address does not.

Which means the useful thing to do first isn't to get a builder's quote. It's to establish which pathway you're on, and what that pathway requires.

Where to start this week

Find your lot size and your zoning. Then measure the existing floor area of your house including the attached garage, carport, deck, patio, pergola, terrace and verandah. That single number tells you more about whether the fast pathway is open to you than any amount of design work will.

Or send us the address and we'll check it for you — lot size, zoning, current standards, and a written document list for whichever pathway applies. That's the free assessment, and for a granny flat enquiry the address really is all we need.

Planning controls change and councils apply their own standards on top of state policy. This is general information current at the time of writing, not planning advice — confirm what applies to your lot before committing to a design.