People almost never arrive with a blank page. They arrive with a duplex plan they found somewhere, printed or on a phone, and they want to know what it would cost to build that on their block. I've stopped answering that question straight away. I ask which way the block falls, where the sewer main runs, and how wide the frontage is once you take the setbacks off. Half the time the plan is fine and needs shifting a metre. The other half it was drawn for a lot two metres wider than theirs and everything after page one is fiction.
So this is the plan-level stuff. Not whether your block suits a dual occupancy at all, which I've written about in is your block right for a duplex, and not attached versus detached or how you title it, which is covered in dual occupancy designs for Illawarra blocks. This is the layout itself, once you know two homes are going on the land.
The garage decides more of the plan than the kitchen does
Every duplex I've built started with the driveways. Two dwellings need two ways in, two lots of parking, and enough room to reverse without a three point turn onto a main road. Get that wrong and no amount of clever planning upstairs saves it.
On a typical suburban lot you've got two workable arrangements. Driveways on the outside edges, garages pushed to each side boundary, front doors sitting in the middle next to each other. Or driveways side by side down the centre, garages back to back, and the entries pushed out to the edges. The first gives each home more privacy at the front door and a bit of dignity from the street. The second saves crossovers and can leave you more usable frontage, but you end up with a fair slab of concrete down the guts of the block and two neighbours parking a metre apart.
Corner blocks dodge most of this, which is why they're worth more for dual occupancy than the extra land alone explains. One address on each street, one driveway each, no shared anything.
What tends to catch people out is the crossover. That's council's call, not yours, and there are usually rules about how close a crossover can sit to an intersection, a power pole, a street tree or another crossover. I've had a plan sent back over a street tree nobody had thought about. Worth checking early, because the driveway positions cascade into everything.
Mirrored, or two different houses?
The standard duplex plan is mirrored. One home flipped across the party wall so the two are identical, just reversed. There's a real reason for it beyond laziness. Mirroring stacks wet areas back to back on the shared wall, so both kitchens, both laundries and both bathroom groups share a plumbing zone. That saves you money in the ground and in the ceiling, and it keeps noisy pipes off bedroom walls.
The catch is aspect. If your block runs east to west, mirroring means one home gets the good northern side and its twin gets the fence. I've seen owners live with that and I've seen them regret it about two winters in. The fix is to hand the plans instead of mirroring them: same footprint, but the living areas and outdoor space of the second dwelling reworked so both sides get sun. It costs more in design and it costs a bit more in plumbing. Worth it or not comes down to one thing: are you selling both, or keeping one?
If you're keeping one, hand them. You'll be the one sitting in it.
The party wall is the bit nobody asks about
The shared wall between two attached dwellings has to do two jobs. It has to hold up fire for a set period, and it has to stop sound. Those requirements sit in the National Construction Code, and if you want to see how the performance side is framed, the ABCB publishes the NCC.
In practice it means a heavier wall system than an internal wall, usually a double stud arrangement or masonry, with no service penetrations cut through it. That last part is where jobs go wrong. Someone wants a power point on the party wall, or a recessed TV cavity, or the plumber decides to run a waste through it, and the fire and acoustic performance you paid for is gone. On my jobs the party wall is drawn with a hatch and a note and nobody touches it without asking.
Design around it. Wardrobes, stairs, garages and hallways all make good buffers on a shared wall. Master bedrooms and media rooms don't.
Single storey or two, and where the living goes
Two storeys is usually the answer on anything under about 600 square metres, simply because a single storey duplex eats the whole block and leaves you two courtyards you can't use. But it's not automatic.
The pattern I build most often is garage, entry, powder room and open plan living on the ground floor, with three or four bedrooms upstairs. It works, it sells, and it's what most buyers around Wollongong expect. The version I like more, on a block with a view or an awkward aspect, flips it. Bedrooms downstairs where it's cooler and quieter, living and kitchen upstairs where the light and the outlook are. It's a harder sell to a valuer and a much better house to live in. If the block falls away to the rear or catches the escarpment, I'll push for it.
Single storey earns its place when the buyer is downsizing, when the block is wide and shallow, or when there's a height control that makes two storeys a fight with council. Ageing in place is a real reason too, and I've done a few where one dwelling is single level for the parents and the other is two storeys for the kids.
Private open space, and the strip nobody wants
Every dwelling needs its own usable outdoor area, and councils set a minimum for it, usually with a minimum dimension attached so you can't count a one metre strip down the side. That dimension is the number that kills bad plans. A courtyard that measures out fine on area but is only two and a half metres deep won't satisfy the control and won't fit a table anyway.
Then there's the side yard. On an attached duplex you get one narrow run down the outside of each home, and it has to swallow the bins, the hot water unit, the air con condenser, the meter box and the clothesline. That sounds like a joke until you're standing there at lock up wondering where the condenser goes. Draw it in at plan stage, on both sides, and check the gate is wide enough to get a bin through.
Storage is the other one. Duplex plans are efficient, and efficient means there's no shed, no roof cavity worth using and often no linen cupboard past the token one in the hallway. I add storage under the stairs on every plan I touch, and I'd rather lose a metre off the living room than hand over a house with nowhere to put the Christmas tree.
What to check before you fall for a plan
If you've got a plan you like, run it past these before you get attached to it.
Take your frontage, subtract both side setbacks, and see whether the plan's overall width fits inside what's left. Eaves and gutters count, so allow for the overhang too. Then look at what your council's controls say about site coverage and landscaped area, because a duplex that fits the width can still fail on how much of the block is under roof and paving.
Check the ceiling heights and the overall building height against your controls, especially if the plan came from Queensland or Victoria. Check whether the plan assumes a flat pad, because on a falling block you're either stepping the slab or bringing in fill, and both change the drawings. And check whether the approval path you're hoping for is realistic. Some duplexes can go through as complying development if they tick every single box, which is faster, and plenty can't. I've written up that fork in CDC vs DA in NSW. The NSW planning system's development assessment pages are a reasonable place to start if you want to read the official version.
Costs are their own conversation, and the numbers are in what it costs to build a duplex in NSW.
Where I'd start
Honestly? Not with a plan. Start with your block's controls and the survey, get someone to draw the buildable envelope on it, and then look at plans that fit inside that shape. It feels backwards because the plan is the fun part, but every duplex job I've seen go sideways went sideways because the drawing came first and the land had to be argued into agreeing with it.
We design and build duplexes and dual occupancies across Wollongong, the Illawarra and the Southern Highlands, and you can see how we run those jobs on our duplex and dual occupancy page. If you've got a block and a plan you're not sure about, send them both through the contact page or call 0434 037 851 and I'll tell you honestly whether it fits.
Want a hand with this at your place? Get a free quote or call 0434 037 851.