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Bay windows are the defining feature of tens of thousands of Irish semi-detached houses built between the 1930s and the 1970s, and they are also the hardest window on the house to replace well. The frame is often holding up the bay roof, the angles are rarely what the drawings say, and a poor job leaves a cold, draughty alcove that lets the whole room down.
This guide explains what bay window replacement costs in Ireland in 2026, why the structure matters more than the glass, the layout and material options, and what a proper installation looks like.
Key Takeaways
- A replacement uPVC bay window typically costs €2,800 to €5,500 supplied and fitted. Aluminium runs €4,500 to €8,000 and AluClad €5,000 to €9,000.
- Most Irish bays are load-bearing. The new frames need reinforced corner posts or steel bay poles to carry the bay roof, and the installer must support the roof while the old frame is out.
- Poor insulation of the bay roof and floor is the usual reason a bay stays cold after new windows. Fix it at the same time.
- Like-for-like replacement is exempt from planning permission. Changing the shape of the bay, or any work on a protected structure, is not.
Types of Bay Window in Irish Homes
- Canted or angled bay. The classic three-section bay with a wide centre pane and two side sections set at 30 or 45 degrees. By far the most common on Irish semis.
- Square or box bay. Side sections at 90 degrees, giving a deeper sill and a boxier look. Common on 1930s houses and some modern builds.
- Bow window. Four, five or more narrow sections forming a gentle curve. Related to, but distinct from, a bay. See our bow windows page for the differences.
- Oriel bay. A bay on an upper floor with no bay below it, supported on brackets or a corbel.
- Two-storey bay. Ground and first floor bays stacked, with the ground-floor frame often carrying part of the load from above.
Why Bay Windows Are Different to Replace
The Frame Is Often Structural
On many Irish houses the bay has its own small pitched or flat roof, and that roof sits directly on the window frames. When the original timber or early uPVC frames were installed, the corner posts were sized to carry it. A standard replacement window is not. A competent installer will either fit steel bay poles (adjustable jacks hidden inside the corner posts) or use a reinforced structural bay post system, and will prop the roof with acrows while the old frames are removed. Skipping this leads to sagging roofs, cracked render and windows that will not open a year later.
The Angles Are Never Standard
Every bay is surveyed individually, because the angles, section widths and projection differ from house to house even on the same estate, and old bays have often moved slightly over the decades. Frames are made to those measurements and joined on site with variable-angle bay couplers.
The Roof, Sill and Floor Come Into It
Bay roofs are frequently uninsulated and their flashings are often tired. The bay floor is usually a suspended timber floor over a vented void. Replacing the windows is the moment to insulate the roof, check the lead or fibreglass flashing and insulate the floor void, otherwise the bay stays the coldest part of the room regardless of how good the new glass is.
How Much Does Bay Window Replacement Cost in Ireland?
Typical 2026 supply-and-fit ranges for a standard ground-floor three-section bay of around 3 metres wide, including structural support, removal of the old frames, new sill and making good:
| Bay window type | Typical fitted price (2026) | Notes |
|---|---|---|
| uPVC, 3-section, double glazed | €2,800 to €4,500 | Most common replacement |
| uPVC, 5-section or bow | €3,800 to €5,500 | More sections, more couplers and openers |
| Aluminium, 3-section | €4,500 to €8,000 | Slim frames, any colour |
| AluClad, 3-section | €5,000 to €9,000 | Timber inside, aluminium outside |
| Bay roof insulation and re-flashing | €600 to €1,500 extra | Strongly recommended at the same time |
| Steel bay poles or structural posts | €150 to €300 each | Usually two, sometimes four |
Upgrading to triple glazing adds roughly 10 to 20 percent. Coloured foils, Georgian bars, a new internal bay board and two-storey bays all add cost. For whole-house budgeting see our window replacement cost guide for Ireland.
Your Options When Replacing a Bay
Material
uPVC is the value choice and suits the majority of estate houses. Aluminium gives the slimmest sightlines, which matters on a bay because there are so many frame joints in view. AluClad gives a timber interior for a period or high-end finish with a maintenance-free exterior. Our material comparison weighs them up in detail.
Opening Configuration
The most popular layout is a fixed centre pane for the view with opening casements on the two angled sides for ventilation. Alternatives include top-hung openers above a fixed lower section, or opening lights in every section. Fewer openers means less cost, more glass and better insulation, but check the room still meets escape and ventilation needs. A fixed picture window in the centre section is the usual way to maximise the view.
Glazing Bars and Style
Georgian bars, astragal bars or plain glass change the character of the house more on a bay than anywhere else, because the bay is the front elevation’s focal point. Look at what the neighbours have done and at our guide to the most popular window styles in Dublin homes.
Colour
White remains the norm on estates, with anthracite grey and cream the most common upgrades. On a bay the frame colour is very visible, so see the guide to choosing window colours and finishes before deciding.
Should You Replace a Bay With a Flat Window?
Occasionally homeowners ask about removing the bay and fitting a flat window in the main wall. It is almost never worth it. You lose floor space, light and the character of the house, the bay roof and floor have to be demolished and the wall rebuilt, and the cost ends up higher than a good bay replacement.
Do You Need Planning Permission?
Replacing a bay window with a new bay of the same size and shape is a repair and does not need planning permission in Ireland. Enlarging the bay, changing its shape or adding a bay where there was none is development and may need permission depending on size and position. Any change to a protected structure or a house in an architectural conservation area needs the planning office’s view first.
What Happens on Installation Day
- Survey. Angles, widths, projection and the condition of the roof, sill and floor are recorded, and the load on the frame is assessed.
- Prop the roof. Acrow props support the bay roof before anything is removed.
- Remove the old frames, sill and any rotten timber, keeping internal boards and plaster where possible.
- Fit the structural posts and new frames, coupled at the correct angles, plumb and level, and fixed to the masonry.
- Transfer the load from the props to the new posts, then remove the props.
- Insulate and seal around every frame, and address the roof and floor insulation if agreed.
- Finish. New sill or sill covers, internal bay board, trims, external sealing and flashing checks.
A three-section bay is normally a one-day job for a two-person team, longer if the roof is being re-covered. Our guide to common window installation mistakes lists the shortcuts to watch for, and how Expert Windows manages installation shows the standard to expect.
Frequently Asked Questions
How much does it cost to replace a bay window in Ireland?
Budget €2,800 to €5,500 for a uPVC bay, €4,500 to €8,000 for aluminium and €5,000 to €9,000 for AluClad, fitted, plus €600 to €1,500 if the bay roof is insulated and re-flashed at the same time.
Are bay windows load-bearing?
Very often, yes. If the bay has its own roof, the window frames are usually carrying it. The replacement must include steel bay poles or structural corner posts, and the roof must be propped during the work.
Why is my bay window area so cold?
Usually because the bay roof and the floor void beneath it are uninsulated, and because old frames leak air at the many joints. New windows plus roof and floor insulation solves it.
What is the difference between a bay and a bow window?
A bay has three (or sometimes five) sections at distinct angles. A bow has four or more narrower sections forming a gentle curve. Bows cost more per metre because of the extra sections and couplers.
Can I replace a bay window with uPVC if the original was timber?
Yes, provided the structural support is designed in. uPVC on its own is not strong enough to carry a bay roof, which is exactly why reinforced posts or bay poles are used.
How long does bay window replacement take?
Typically one day for a standard ground-floor bay, two days if the roof is being insulated and re-covered or the bay is two storeys.
Final Verdict
Replace a bay window properly and it becomes the warmest, brightest part of the room again and lifts the whole front of the house. Get the structure, the angles and the roof and floor insulation right, and choose the material and layout that suit the house rather than the cheapest quote. Expect €2,800 to €5,500 for uPVC and more for aluminium or AluClad.
Expert Windows surveys, manufactures and fits made-to-measure bay windows in uPVC, aluminium and AluClad across Dublin and Ireland, with structural support included in every quotation. Request a free survey and quote.
