There is no shortage of granny flat floor plans available to look at. Most of them are fine as drawings, however the problem is that a small dwelling has no room for a mistake.
In a four bedroom house, a badly placed hallway is an annoyance. In an 80 square metre dwelling, it is the difference between two comfortable bedrooms and two bedrooms nobody wants to sleep in. So rather than another gallery of plans, here is what actually makes a small dwelling work, whether you are building a granny flat on the back of an existing house, buying a portable or modular home, or converting space under a raised Queenslander.
Decide who lives there before you look at a single plan
This is the step almost everybody skips, and it changes the design more than anything else.
A dwelling for an ageing parent is not the same building as a dwelling for a tenant, even at the same size. For a parent, the design priorities are level entry with no steps, doorways and hallways wide enough for a walker or a wheelchair later on, a hobless shower with room for someone to assist, lever taps, and a bathroom you can turn around in. Getting those right at design stage costs almost nothing. Retrofitting them costs a bathroom.
For a tenant, the priorities are different. Privacy from the main house matters far more, along with a genuinely separate entry, some outdoor space of their own, somewhere to park, and enough storage that they are not stacking boxes in the second bedroom.
For an adult child moving back home, the answer usually sits between the two, with a stronger emphasis on separation and noise.
Design for one of these properly and it will still work reasonably for the others. Design for none of them and it works for nobody.
One bedroom or two, and what that costs you
At an 80 square metre cap, this is the decision that shapes everything else. A one bedroom layout is comfortable. You can have a proper bedroom, a decent bathroom with a separate laundry, generous open plan living, and real storage. It suits a parent, a couple, or a single tenant, and it usually feels larger than the floor area suggests.
Two bedrooms at the same area means everything else shrinks. The second bedroom is often a genuine single room rather than a double, the living space gets tighter, and the storage is the first thing sacrificed. It is absolutely doable and plenty of two bedroom flats work well, but it needs to be a deliberate trade rather than something you discover at the end.
The honest question is whether the second bedroom earns its space. If it is for occasional guests, a well designed one bedroom plan with a study nook or a sofa bed usually delivers a better result. If it is for a second tenant or a child, it is worth the compression.
Circulation is the enemy
Every square metre of hallway in a small dwelling is a square metre taken from somewhere useful. Good small plans have almost no corridor. The entry opens into the living space, the bedrooms come off that space or off a very short passage, and the wet areas group together on one side so the plumbing runs short and the walls do double duty. Grouping the bathroom, laundry and kitchen against a shared wall saves both floor area and cost.
The other rule is that rooms should not be shaped by leftovers. If the bedroom is the space remaining after everything else was placed, it will end up a size and proportion that fits a bed awkwardly. Set out the bed, the wardrobe and the circulation around them first, then let the rest follow.
Front door positioning
The most common liveability failure in secondary dwellings is not the floor plan. It is the relationship between the two households.
Three things matter. The first is a separate approach, so whoever lives in the flat can come and go without walking past the main house windows or through the family’s outdoor space. The second is sightlines, meaning the flat’s living room windows should not look directly into the main house bedrooms, and vice versa. The third is acoustics, because in a detached flat two metres from a bedroom wall, an air conditioning unit or a late television is a real problem.
None of this requires much space. It requires the two buildings to be positioned with each other in mind, which is difficult to do if the flat was selected as a plan before anyone looked at where the existing house sits.
Orientation is a compliance issue
Small buildings heat up fast so there is a lot of external wall and roof relative to the volume inside, so a poorly oriented granny flat in Brisbane is genuinely unpleasant by mid afternoon in summer.
The basics still apply. Put living areas and their glass to the north where you can shade them with an eave. Keep west facing glass small and shaded, because that is the afternoon heat load. Get windows on two sides of the living space so it cross ventilates, which in this climate does more for comfort than any amount of mechanical cooling.
Since May 2024 all new dwellings in Queensland have had to meet the 7 star NatHERS standard, and a secondary dwelling is a new dwelling. Orientation, glazing, shading and insulation are what get you there. Design with them in mind and the rating looks after itself. Leave them to the end and you pay for it in upgraded glazing to drag the number over the line.
Covered outdoor space is the cheapest area you will ever add
This is the single best move available on a small dwelling in this climate. A covered deck or patio gives you living space that works nine months of the year, costs a fraction of enclosed floor area, and is generally treated differently to gross floor area when it is uncovered, which matters when you are working to a size cap. Even where a roofed structure does count, the effect on how big the dwelling feels is out of all proportion to the money spent.
A three by four metre covered deck off the living room turns a compact flat into something that lives generously. It also gives a tenant their own outdoor space, which solves part of the privacy problem at the same time.
Storage is the thing people complain about afterwards
A granny flat has no garage, usually no roof space worth using, and often no shed. Whatever storage exists has to be designed in.
Built in wardrobes rather than freestanding, a linen cupboard that is actually deep enough, a laundry that doubles as a utility store, and somewhere for a vacuum, a toolbox and a suitcase. On a two bedroom plan this gets forgotten first, which is exactly why the second bedroom ends up full of boxes.
Ceiling height helps here too. At 2.7 metres you get taller cupboards, higher windows for light and ventilation, and a room that feels considerably larger for very little extra cost.
Portable, modular and transportable homes
These get grouped together but the design conversation is the same. You are choosing an existing plan and adapting the site to it, rather than designing to the site.
That is a good trade on a flat, regular, easily accessed block. The build is fast, the price is more certain, and factory conditions produce a consistent result. It is a poor trade on a sloping block, a narrow block, a site with significant trees, or anywhere a truck and crane cannot easily get to. Delivery access is the constraint people discover last and it is not a small one.
A few things worth knowing before you commit. Transport limits mean modules come in set widths, so wider open plan spaces are made by joining modules and the join has to land somewhere sensible in the plan. Site works, footings, connection to services, and the crane are usually quoted separately to the building, and on a difficult site they can rival the cost of the unit itself. Most importantly, a portable or modular dwelling is still a dwelling. It needs the same planning consideration and the same building approval from a certifier as anything built in place. Buying it as a product does not change the approval pathway.
Building underneath an existing house
In Brisbane there is a fourth option that gets overlooked. On a raised or already high set house, the space underneath can often become the secondary dwelling, and on a house that is going to be raised anyway it is worth considering at the same time.
The design questions are different. You are working with an existing floor structure and stumps, so ceiling height, natural light and ventilation are the constraints rather than floor area, and the entry and privacy questions get easier because the two dwellings are already stacked rather than facing each other. Whether it is possible depends on the existing height, the fall of the land, and what the planning scheme allows on that particular property, particularly in character areas. It is worth asking the question before assuming a detached flat in the back yard is the only option.
The design is only as good as the block it was drawn for
Everything above assumes someone has established what the site will actually take. Boundaries, levels, services, overlays and the applicable planning pathway all set the envelope a design has to work within, and they are the reason otherwise good plans get redrawn.
If you have not covered that ground yet, start there. Our guide to granny flat rules in Brisbane sets out what you can build without a development application and what triggers one, and it is worth reading before you get attached to a floor plan.
Then design for the person who is going to live in it. That part is not complicated, and it is what makes the difference between a granny flat that gets used and one that sits empty.