Dual Occupancy and Multi-Generational Homes in Australia: Windows, Privacy, Acoustic Separation and Fire Compliance

Dual occupancy and multi-generational housing are now the fastest-growing segment of Australian residential construction. Rising land costs, multigenerational family living, and planning reforms that permit secondary dwellings in more zones have pushed thousands of projects into the space between a house and an apartment building — and windows sit at the centre of nearly every dispute that follows.

Two dwellings on one lot create three problems that do not exist in a single house: neighbours can see into each other, they can hear each other, and a fire in one can reach the other through an unprotected opening. All three are solved at the window, and all three are decided long before anyone chooses a frame colour. This guide sets out the privacy, acoustic and fire requirements that govern glazing in dual occupancy and multi-generational homes in Australia, and the specification strategies that satisfy them without turning the building into a bunker.

Quick answer: the four glazing decisions in every dual occupancy

1. Overlooking is regulated by geometry, not goodwill. Sill heights, screening and setback rules (commonly a 9 m radius and 45° cone from habitable room windows) determine what glazing you can have on each facade.
2. Acoustic separation is a window problem before it is a wall problem. A party wall at Rw 50 is defeated by two open windows 3 m apart.
3. Fire separation restricts openings near the boundary. Where a wall sits on or near a boundary, unprotected windows may not be permitted at all.
4. Accessibility obligations apply to the secondary dwelling. A granny flat intended for an elderly parent should meet liveable housing provisions — 820 mm clear openings, low thresholds, reachable hardware.

What counts as dual occupancy in Australia

The terminology varies by state, and the compliance pathway follows the terminology.

The classification matters because fire and acoustic provisions differ between Class 1a and Class 1b, and because state planning schemes impose their own setback and overlooking tests on top of the NCC.

Overlooking: the geometry that sets your window schedule

Privacy between dwellings is regulated by a simple geometric test in most Australian planning schemes, and it is expressed in terms of what a person can see from a habitable room window.

The common form of the test — used in various guises across Victoria’s ResCode, NSW’s planning controls and equivalent instruments in other states — is:

• A 9 m radius measured from the window, within which overlooking into a neighbouring habitable room window, terrace, balcony or private open space is considered.
• A 45° cone projected horizontally from the window plane, defining the area of potential view.
• A screening standard that either raises the sill to 1.7 m above floor level, uses obscure glazing, or provides permanent screening (louvres, fins, planter boxes, perforated panels) so that sightlines are blocked.

In practice, this produces three design responses:

Raise the sill. A sill at 1.7 m above floor level prevents a standing person from seeing down into a neighbouring space. In a bedroom this is workable — a high window still delivers light and ventilation. In a living room it is not, which is why the second option exists.

Obscure the glass. Translucent or obscure glazing (typically acid-etched, sandblasted or a rolled pattern) admits light while destroying the view. The limitation is that occupants then have no view at all, and obscure glazing that can be opened defeats the purpose — so the operable sash needs the same treatment, or the window must be fixed with a separate high-level operable panel.

Screen externally. Louvre blades, vertical fins, slatted panels and deep reveals block the sightline while retaining a view of the sky and, at the right angle, of the garden. This is the best architectural outcome and the most common solution in good dual occupancy design, because it lets both dwellings have real windows.

The detail that catches people out is oblique overlooking — the diagonal view from one dwelling’s window across to the other’s deck, which often falls outside a simple 45° check but is the view that actually causes complaints. A sightline drawn in plan from the corner of the window, at 45° to the window plane, on both sides, is the only reliable way to test it.

Acoustic separation: where dual occupancy actually fails

The NCC requires separating walls and floors between sole-occupancy units to achieve a defined acoustic performance — commonly Rw 50 for walls and Rw 45 for floors, with the requirement expressed as a laboratory rating for the element. Designers spend a great deal of effort on the party wall and then ruin it with windows.

Noise between two dwellings travels by four paths, and windows affect three of them:

Direct path through open windows. Two habitable room windows 4 m apart, both open on a summer evening, provide an almost free acoustic path. Measured insertion loss of an open window is close to zero. This is the dominant cause of neighbour noise complaints in dual occupancies, and no wall specification can compensate.

Flanking through the facade. Sound travels along the outside of the building and re-enters through the other dwelling’s glazing. Deep reveals, staggered facades and screens interrupt this path.

Structure-borne and gap leakage. Sound leaks through gaps around frames, through unsealed penetrations and via the frame’s connection to shared structure. A 3 mm gap around a sash can reduce a window’s effective rating by 10 dB.

The practical design rules that work:

• Stagger or offset windows. Do not place habitable room windows of the two dwellings directly opposite each other, and avoid facing bedroom windows across a narrow gap. An offset of even 1.5–2 m breaks the direct path substantially.
• Specify asymmetric laminated glazing on the most exposed facade. For the dwelling facing the neighbour, a 6.4 mm laminated / 12–16 mm argon / 6 mm unit achieves Rw 36–40; a 10.8 mm laminated build-up reaches 42–45. Laminated glass outperforms monolithic toughened of the same thickness at equivalent weight because the interlayer damps coincidence resonance.
• Use different glass on each side. Two different build-ups (different thicknesses and gaps) avoid coincident resonance and typically give 2–4 dB better performance than identical windows facing each other.
• Seal the perimeter properly. Compression gaskets, a continuous backer rod and sealant at the frame-to-reveal junction, and quality weather seals at the sash. Acoustic performance is a sealing problem as much as a glass problem.
• Treat the door. A dual occupancy almost always has an external door from one dwelling near the other. Solid-core doors with acoustic seals and an automatic drop seal are necessary if the door faces a shared outdoor area.
• Consider mechanical ventilation instead of open windows. Where privacy and noise both matter, an HRV or a quiet ceiling fan allows windows to stay closed at night — often the single most effective acoustic measure available.

Fire separation and openings near the boundary

Fire performance between dwellings is a code requirement, and glazing is where it is most often compromised.

For an attached dual occupancy, the separating wall must achieve the required fire-resistance level, and openings in that wall are generally not permitted — which means no windows between dwellings, ever. Windows in the external walls of each dwelling are then assessed against the boundary setback rules: where an external wall is on or close to a boundary (commonly within 900 mm, with variations by state and class), openings are restricted or must be protected with fire-rated glazing and construction.

Fire-rated glazing is a specialist product line: it is not ordinary toughened or laminated glass, and its rating depends on the whole assembly, including the frame. If a certifier requires a −/60/60 or −/30/30 rating for a window opening, the frame must be a tested fire-rated system with the correct glazing, installed exactly as tested. Substituting a standard window with thicker glass is a compliance failure that will be picked up at occupation certificate stage.

Three practical points:

• Locate windows away from the boundary. A window facing the street or the rear yard is far cheaper than a fire-rated window facing a side boundary.
• Protect with a blade wall or return. A non-combustible blade wall projecting from the facade can protect an opening without fire-rated glazing in many configurations.
• Check egress. Every dwelling needs a means of escape from each bedroom, which in practice is an openable window with adequate clear opening dimensions. A fixed or high-sill window may not satisfy egress; check the clear opening against the NCC’s requirements for the specific room.

Multi-generational design: accessibility and privacy for different generations

A granny flat or dual-key arrangement is often built for a parent, an adult child with a disability, or a family supporting shift workers and young children. Each case changes the glazing brief.

Accessibility. The secondary dwelling should meet liveable housing provisions if an occupant has mobility limitations: 820 mm clear door openings, thresholds no more than 5 mm with a bevel, lever handles at 900–1,100 mm, and at least one step-free entry. Windows need reachable controls — a high awning sash with a winder at 1,800 mm is unusable for a seated occupant. Specify sliding or casement sashes with handles in the accessible zone, or motorised operators.

Acoustic privacy inside the dwelling. A multi-generational home needs acoustic separation between the generations as much as between dwellings. Bedroom windows facing a shared courtyard, a teenager’s bedroom above a grandparents’ living room, a baby’s room next to a home office — these are internal acoustic problems solved with laminated glazing, solid-core doors and thoughtful window placement.

Temperature zoning. Older occupants typically want warmer rooms. If the secondary dwelling shares a glazed wall with the main house or has a large west-facing window, its temperature swings more than the rest of the building and the occupant runs a heater in a room that is simultaneously overheating in the afternoon. Solar control glass (SHGC 0.25–0.35) on west and north glazing, plus thermally broken double glazing, reduces the swing and the running cost.

Security and independence. A secondary dwelling should have its own secure entry, its own lockable windows with keyed hardware, and security screens on ground-floor openings if the occupant is vulnerable. Screens tested to AS 5039 and installed to AS 5041 give both physical security and the ability to leave windows open at night — which for an elderly occupant is a genuine quality-of-life improvement.

Daylight and outlook. Secondary dwellings are often squeezed into the least favourable part of the site — behind a garage, under a main roof, against a boundary. Windows become the only source of daylight, so sill heights, glazing area and orientation have to be fought for at design stage. A high-level clerestory or a lightwell with fixed glazing can rescue a room that would otherwise be permanently dim.

Servicing, drainage and maintenance between dwellings

Two dwellings on one lot share services and create new maintenance obligations:

• Gutter and downpipe access. A window that opens over a shared gutter or a boundary makes gutter cleaning difficult. Consider fixed glazing or an inward-opening sash where access is constrained.
• Drainage across the boundary. A window sill that discharges water onto a neighbour’s path is a dispute waiting to happen. Sills should drain to the dwelling’s own side.
• Cleaning access. Windows on a boundary side that cannot be reached from either property will stay dirty. Specify tilt-in or reversible sashes so both faces can be cleaned from inside.
• Condensation in the secondary dwelling. Small dwellings with high occupancy have high internal moisture. A bathroom without adequate ventilation will push moisture into the bedroom and condense on the coldest glazing. Double glazing plus a compliant exhaust fan solves this.

Budget expectations

Specification checklist

1. Draw the 9 m / 45° sightline cones for every habitable room window before fixing the schedule.
2. Decide privacy strategy per window — sill height, obscure glass, or external screen — and record it on the drawings.
3. Offset facing windows between dwellings wherever possible; never place bedroom windows directly opposite each other.
4. Specify laminated acoustic glass on facades that face the neighbouring dwelling, with different build-ups on each side.
5. Confirm the separating wall has no openings, and check every boundary-facing opening against fire separation requirements.
6. Confirm egress window sizes in each bedroom.
7. Apply liveable housing provisions to the secondary dwelling: 820 mm clear, 5 mm thresholds, accessible hardware.
8. Specify solar control glass on west and north facades to reduce temperature swing in the smaller dwelling.
9. Use AS 5039-tested security screens on ground-floor openings of the vulnerable dwelling.
10. Specify tilt-in or reversible sashes for boundary-facing windows that cannot be cleaned externally.

Frequently asked questions

How far apart do windows need to be to avoid overlooking in a dual occupancy?

Most Australian planning schemes use a 9 m radius combined with a 45° cone from the window plane. If a neighbouring habitable room window, balcony or private open space falls inside that area, overlooking controls apply and you must either raise the sill to 1.7 m, use obscure glazing, or install permanent screening. The 9 m and 45° figures are common but not universal — always check the applicable state or council planning scheme.

Do both dwellings in a dual occupancy need acoustic-rated windows?

They need acoustic separation, which is achieved through a combination of wall performance, window performance and geometry. The party wall typically must achieve Rw 50 under NCC provisions, but the windows are often the weakest path, particularly when both dwellings open windows onto the same courtyard. Specify laminated acoustic glazing on the facades facing the other dwelling and offset the windows wherever possible.

Can a dual occupancy have windows facing each other across a courtyard?

Yes, provided privacy is addressed. The usual approach is to offset the windows so there is no direct line of sight, use obscure glazing or external louvre screens on the most exposed openings, and raise sills on bedrooms and bathrooms. Where the courtyard is small, a combination of deep reveals, planter boxes and vertical fins can preserve usable windows on both sides.

Are fire-rated windows required between two dwellings?

Between attached dwellings, the separating wall generally cannot have openings, so there are no windows between them. Where an external wall of one dwelling is on or close to a boundary — commonly within 900 mm — openings may be restricted or must be protected with a tested fire-rated glazing assembly, including a fire-rated frame. Windows facing the street or rear setback are normally unaffected.

What glass is best for a granny flat next to the main house?

For a secondary dwelling close to the main house, specify thermally broken aluminium frames with double glazing (argon, low-e) for comfort and condensation control, and laminated acoustic glass on the facade facing the main dwelling’s living areas. Add solar control on west-facing glazing to reduce afternoon heat, and use AS 5039-tested security screens on ground-floor openings if the occupant is elderly or vulnerable.

Conclusion

Dual occupancy glazing is decided by geometry and code before it is decided by aesthetics. Get the sightline cones and setbacks right, protect the boundary openings against fire, offset and acoustically upgrade the facing windows, and detail the secondary dwelling for accessibility. Do those things and two dwellings on one lot can each have generous, usable, private windows. Skip them and the result is a building where every window is either permanently covered or the subject of a complaint.

MEICHEN manufactures AS 2047 and AS 4284 tested aluminium window and door systems with thermally broken profiles, 2.0 mm wall thickness, laminated acoustic double glazing achieving up to 45 dB, and 3000-hour salt-spray-tested finishes. Systems for dual occupancy projects — from sliding and casement ranges to lift-slide doors and high-sill privacy windows — are produced at the Zhaoqing facility in Guangdong and supplied with test documentation for Australian certifiers.