A bay window adds floor space, light and street presence, but it costs more than a flat window because it needs its own support, roof and flashing. Here are indicative Australian bay window costs, the price drivers and the trade-offs.
Key Takeaways
- Indicative supply and install pricing for an aluminium bay window runs from $4,000 to $12,000, with new structure and roofing pushing larger projects higher.
- Glass area, glazing type, number of opening sashes and whether the wall needs a new lintel are the main cost drivers.
- A bay has three angled panels, a bow has four or more in a curve, a box has square returns and a raked window follows a pitched ceiling.
- Double glazing and thermally broken frames matter more on a bay because there is more glass and frame exposed to the weather.
- Replacing a rotting timber bay with aluminium in the same opening avoids most of the structural cost.
Bay, Bow, Box or Raked
A bay window is a three-panel unit that projects from the wall, usually with a large fixed centre and two sides angled at 30 or 45 degrees. The alcove it creates suits living rooms, dining areas and main bedrooms.
A bow window uses four to six narrower panels in a gentle curve, which looks softer but adds cost with every mullion. A box window projects at 90 degrees with square returns, suits contemporary homes and is the simplest to roof and flash.
A raked window does not project at all. It is a fixed window with a sloping top edge made to follow a pitched or cathedral ceiling. Arch Aluminium Windows and Doors manufactures both bay and raked windows in Melbourne, each made to the exact opening and roof angle.
Common Bay Configurations
The standard arrangement is a fixed centre panel with two opening sides that handle ventilation.
- Fixed centre with awning sides: awnings can stay open in light rain, which suits Melbourne’s changeable weather.
- Fixed centre with casement sides: casements open fully and catch cross breezes.
- All fixed: the cheapest and best thermal performer, but no ventilation.
Widths run from about 1,800mm to 3,600mm or more, with a projection of 400mm to 600mm. The bay and rake window range is built to order in Alspec systems, so widths, angles and sash types follow the opening rather than a catalogue.
Structure and Roofing Requirements
A bay carries its own weight plus wind and glass loads past the wall line, so it needs support underneath and a weatherproof cover on top.
- Base: cantilevered floor joists, a small footing or steel brackets into the wall frame.
- Lintel: a wider opening needs a new or larger lintel, often with engineer sign-off.
- Roof: a small hip, a flat skillion or a tuck under the existing eave.
- Insulation: the seat, cheeks and roof must meet NCC energy provisions.
In Victoria, adding a projecting bay to an existing wall generally needs a building permit. Replacing a bay in the same opening usually does not, but confirm with your council or a private building surveyor.
Indicative Bay Window Costs
These are indicative Australian prices for aluminium units, supplied and installed. Site conditions move them, so get a measured quote before budgeting.
- Replacement bay in an existing opening, single glazed, 1,800mm to 2,400mm: $4,000 to $7,000.
- Same unit double glazed with thermally broken frames: $6,000 to $10,000.
- New bay cut into a wall, including lintel, seat, roof and flashing: $9,000 to $16,000.
- Large bow or multi-panel bay over 3,000mm: $12,000 upward.
Each opening sash adds hardware, seals and labour, and limited access or two-storey work adds more. Timber frames typically cost 30% to 60% more than aluminium and need repainting every few years.
If the bay is part of a whole-house replacement, our guide to the cost of replacing aluminium windows covers per-window pricing and what affects the total.
Glazing and Energy Performance
A bay has more glass and frame than a flat window and sits in the weather on three sides, so glazing is the biggest influence on comfort. Single glazing from 4mm to 12.4mm suits mild aspects and tighter budgets. Double glazing cuts heat loss by roughly half and noticeably reduces road noise.
Thermally broken frames stop the aluminium conducting heat straight through the frame. Check the WERS rating of the complete window rather than just the glass, and aim for a U-value under 3.0 on a south-facing or cold room. In bushfire-prone areas the unit must also meet the site’s BAL rating.
The centre pane does most of the work. A large fixed window panel with Low-E double glazing gives the best balance of light, view and performance.
Replacing a Timber Bay
Many older Melbourne homes have timber bays with rotting sills, sticking sashes and single glazing. Replacing the window unit while keeping the seat and roof avoids most of the structural cost.
The installer checks the seat and header for rot, makes an aluminium unit to the opening, removes the old frame and redoes the flashing and trims. Most like for like replacements take one to two days. If the seat is soft, budget for carpentry first.
Pros and Cons
Pros:
- Extra usable floor space for seating, a desk or a reading nook.
- Light from three directions in rooms with only one external wall.
- Strong kerb appeal and a point of difference at resale.
- Better cross ventilation than a flat window.
Cons:
- Higher cost, and much higher if new structure and roofing are needed.
- More heat loss and gain unless glazing is specified well.
- More flashing junctions, so installation quality matters more.
- Window coverings need custom solutions to suit the angles.
FAQ
How much does a bay window cost in Australia?
An aluminium bay replacing an existing one costs roughly $4,000 to $10,000 supplied and installed, depending on size and glazing. A new bay cut into a wall usually lands between $9,000 and $16,000.
Do I need a permit to add a bay window?
In Victoria, adding a projecting bay to an existing wall generally needs a building permit because it changes the structure. A like for like replacement usually does not, but check with your council or building surveyor.
What is the difference between a bay and a bow window?
A bay has three panels with angled sides, giving a defined alcove. A bow has four or more panels in a curve, which looks softer and lets in more light but costs more because of the extra frames and glass.
Can a bay window be double glazed?
Yes, and it should be where the budget allows. A bay exposes more glass to the weather than a flat window, so double glazing and a thermally broken frame make a bigger difference to comfort and energy bills.
Conclusion
A bay window earns its extra cost when the room will use the space and light it creates and the structure and glazing are done properly. Confirm the support, roofing and permit position before you commit. For a measured price on a bay or raked window in aluminium, you can request a free quote from Arch Windows.
