A bay projects from the wall and is carried on its own foundation or, on many houses, partly by the window frame itself. That last possibility is what makes bay replacement different from every other window job: take the frame out carelessly and the brickwork or the little roof above has nothing holding it up.
The shapes and what they are called
A splay or canted bay has angled returns and a flat front, and it is the standard Victorian and interwar shape. A box bay has square returns. A bow bay is a shallow curve of several narrow lights. An oriel projects at first floor with nothing beneath it, carried on corbels or brackets. The shape decides how many corner posts there are and therefore how much frame you are paying for relative to glass.
Bay windows prices: why replacement is not a like for like swap
On a great many houses, especially interwar semis, the bay's corner posts carry the brickwork, the tiled bay roof or the bay above. Competent installers deal with this by propping the structure before the old frame comes out, or by fitting structural posts within the new frame. That is why a bay costs more than the same area of flat window, and why a quote that does not mention support is a quote from someone who has not looked.
The roof over the bay
Most ground floor bays have a small roof of their own, tiled, leaded or felted, tucked under the wall above with a flashing. It is small, it is old, and it is often the most neglected roof on the house. Failures show as damp on the wall inside the room above the bay rather than as a drip, which is why they run for years. If the window is coming out, that is the moment to look at it, because the access is there.
1930s bay window replacement cost, and what you lose in character
A 1930s bay with slim timber sections and a tiled roof carries much of the character of the elevation, and replacing it with wide UPVC sections changes the house noticeably. Slimmer aluminium or timber sections cost more and keep the proportions. In a conservation area or on a listed building the choice may not be yours, so check the position before ordering anything.
Questions people ask about bay window types
Why is the cost of a bay window so much higher?
More corner posts and frame per unit of glass, more complex fitting, and on many houses temporary structural support while the old frame is out. The bay roof above it is often part of the job too.
Do bay windows hold up the wall above?
On a good many houses the corner posts carry load from the brickwork or the bay roof. It has to be assessed before removal and supported during it.
Should I replace the bay roof at the same time?
If it is old and the access is already there, almost always. A failed bay roof shows as damp inside the room above rather than as an obvious leak, so it is worth inspecting while the window is out.
Can I keep the original timber bay?
Often yes. Sound timber can be repaired and draught proofed, and secondary glazing behind it gives much of the thermal and acoustic benefit while keeping the elevation, which is frequently the right answer on a period or listed house.