A window well cover is not required by any building code we know of in Canada. It is a choice. Here is how to make it.
Four reasons to fit one
1. The well fills with rain or snow. An open well is a bucket set against your foundation. Everything that falls on it stays there until it drains or soaks away, and a wooden sill sitting in water rots from the outside face where you cannot see it.
2. Somebody could fall in. A well is a hole in the ground, often close to a walkway, often invisible in long grass or fresh snow. This is the reason most people finally buy one — a child, a dog, a delivery driver cutting across the lawn.
3. It fills with leaves and rubbish. Leaves, grass clippings and blown litter collect in a well and pack down at the base, which blocks the drainage you paid for and holds moisture against the window.
4. Animals get in and cannot get out. Mice, toads, squirrels and the occasional cat drop into a well and cannot climb the steel. It is unpleasant to deal with and worse to ignore.
Two situations where you can skip it
The well is under cover already. If the well sits beneath a balcony, a deep eave or an enclosed porch, it may already be taking very little weather. Check it after a heavy rain before spending money.
You need the light and the well drains well. On a dark basement where the well is fenced off or otherwise not a hazard, and where the base is free-draining gravel tied into weeping tile, an open well is a reasonable choice. A clear panel gets you both, but it is not free.
What a cover will not do
This is worth being blunt about, because covers get sold as a fix for wet basements and they are not one.
A cover stops water arriving from above. It does nothing about water arriving through the soil from below, which is what happens when the base of the well is packed clay instead of clear stone. If your well fills up days after the rain has stopped, or in spring with no rain at all, a cover will not touch it — the problem is underneath and it needs digging.
Fix the drainage first. Then a cover keeps the fixed well working.
If the window is a bedroom exit
You can still fit a cover, and most people should. The rule is that it has to open from inside the well, by hand, without tools or a key. That pushes you toward a lighter cover on that particular window — somebody may be pushing it up from below. See bedroom window requirements.
What it costs
Steel covers start at $350 and aluminum at $520, made to your well's size. A moulded plastic dome from a hardware store costs a good deal less and lasts a good deal less — the comparison is here.
One thing worth knowing if you are buying more than one: shipping is charged once per order, not per item. Four covers cost the same to ship as one.
Short answer
If the well is open to the sky and anywhere near where people walk, yes. If it is sheltered, drains properly and nobody can step into it, you can leave it — but check it after the next big rain before you decide.
When you are ready, all 42 sizes are here, made in Canada, and fitting one takes about twenty minutes.