If you have started looking into granny flats, you have probably seen two very different offers. One is a kit or prefab granny flat with a set model and a headline price. The other is a custom build shaped around your block. Both can end up as a good home. They just get there in different ways, and the right choice depends on your site, your budget, and how you plan to use the space long term.
What a granny flat kit actually is
A granny flat kit, sometimes sold as a kit home granny flat, is built from a fixed design. Some arrive as flat-packed panels you assemble on site, others turn up mostly finished on the back of a truck. The appeal is easy to see. You pick a floor plan, you get a clear starting price, and the timeline can be shorter because a lot of the work happens off site.
That model works best on a flat, easy block with good access. The catch is that the advertised price is usually for the module on its own. Site costs, groundwork, connections and finishes often sit outside it.
Where the kit price stops and your costs start
The number on the brochure is rarely the number you pay. On most blocks you still need to budget for:
- Site prep and a slab or footings, which get pricier on a slope
- Connecting water, sewer, stormwater and power to the dwelling
- Council or certifier approval for your specific site
- Crane or truck access, which is tricky on tight Illawarra blocks
- Driveways, paths, landscaping and any retaining
None of that is a reason to rule out a kit. It is a reason to ask for the all-in figure before you compare anything. As a rough guide, a finished granny flat in NSW tends to land somewhere in the low hundreds of thousands once site works are included, and a kit shell is only part of that. Treat that as a starting point, not a quote.
When a custom granny flat is the better call
A custom granny flat is designed around what you actually have: the fall of the land, where the sun hits, the trees you want to keep, and how the new dwelling sits next to the main house. That flexibility matters most when the block is sloped, narrow or has awkward access, which describes a lot of sites across Wollongong and the Southern Highlands.
Custom also wins when the granny flat is a permanent home rather than an occasional guest room. If a parent is moving in for good, or you plan to rent it out long term, the extra thought on layout, storage, ceiling height and materials pays off every day someone lives there.
Quality and how it holds up
Kit quality varies a lot between suppliers, so it is worth reading exactly what is and is not included. Standard fittings, thinner wall build-ups and set finishes keep the price down, which is fine for some uses and limiting for others.
With a custom build you choose the materials and the standard, and the trades are working to your site rather than a catalogue. Done properly, both approaches meet the National Construction Code. The difference usually shows up in the details: how doors and windows are flashed, how the wet areas are waterproofed, and how the whole thing wears after ten years.
Fitting it to your block and the rules
In NSW a secondary dwelling is capped at 60 square metres of internal floor area, and many granny flats can go through as complying development rather than a full DA. Whether yours qualifies depends on your land size, zoning, easements and setbacks, so confirm it with your council or a private certifier before you commit to a design or a kit.
This is where a builder who handles design, approval and construction under one roof saves you the runaround. At Ovas we look at your block first, tell you honestly whether a granny flat kit or a custom build suits it, and put the real all-in numbers in front of you before you decide either way.
Common questions
- Are kit granny flats cheaper than custom builds?
- Often the headline price is lower, but it usually covers the module only. Once you add site works, a slab, connections and approval, the gap narrows. Ask for an all-in figure before you compare, and treat any number as a starting point, not a quote.
- What is the maximum size for a granny flat in NSW?
- A secondary dwelling is capped at 60 square metres of internal floor area. Whether yours can go through as complying development depends on land size, zoning and setbacks, so confirm it with your council or a private certifier for your own block.
- Is a kit granny flat good enough to live in permanently?
- It can be, if the supplier's inclusions and materials suit long-term living. For a permanent home we usually lean toward custom, because layout, storage and durability matter more when someone lives there every day.
- Does a sloped or tight block rule out a kit?
- Not always, but slopes and difficult access add crane, footing and site costs that eat into a kit's price advantage. On tricky Illawarra and Southern Highlands blocks, a custom build designed around the site is often better value.