A Bump Out Bay is the shallowest of the Bay shapes. A square projection and a sill, without the depth — or the roof and header work — of a full Box Bay.
It is the smallest move in the set that still reads as a Bay from the street. For a lot of walls that is exactly the right amount, and it is often what somebody means when they say they want the window to “come out a bit”.
The same square shape, taken out far enough to need a roof over it and real work behind it.
The shallow version. You keep the square projection and the sill; you drop the depth and the build.
Depth is the whole difference, and depth is what the work behind a Bay is mostly for. All six Bay and Bow shapes are drawn out together, with the size ranges we build to and what drives the cost.
When you want the wall to change and you do not want to rebuild it. A Bump Out gives a flat window a sill, a bit of shadow on the elevation and more light into the room, and it asks less of the house than any other shape here.
What it will not give you is a place to sit. If the picture in your head is a window seat with cushions on it, that is a Box Bay or a Hat Box, and it is a bigger job for a good reason — the depth that makes the seat is the depth that needs the roof and the header. A Hat Box Bay goes the other way entirely, adding glazing in the flanks as well as across the front.
Which of those your wall can take, and what it would cost, is a survey question rather than a catalog one. One of our Bays is photographed step by step if you want to see what the deeper end of the range actually involves.
The shallowest of the Bay shapes. It projects squarely out from the wall and gives you a sill, but without the depth of a full Box Bay — and so without the roof and the header work that the deeper shapes need. It is the smallest gesture in the set that still counts as a Bay.
Depth, and what that depth costs you. A Box Bay projects far enough to give you the deepest shelf of any Bay, and paying for that means a roof over the top and real structural work behind it. A Bump Out makes the same square shape much shallower, so you get the sill and the light without the build.
It is usually the least involved of the shapes, and the work behind a window is a large part of what a Bay or Bow costs. But the price of any of them turns on the opening, the wall and what is above it, not on the shape alone. The honest answer is that nobody can price it properly without seeing the wall.
A survey decides that, not the shape. What is true is that it avoids the roof and the header work a full Box Bay needs. But any opening cut into a wall that carries load has to be spanned and borne, and whether yours does is a question about your wall. We look before we say.
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