Cutting a New Opening vs Enlarging an Existing Window

Nearly everyone arrives at this decision with the same assumption: that using the hole already in the wall must be cheaper than making a new one.

It is a reasonable assumption. It is also, for the specific case of a Calgary basement window, wrong more often than it is right — and the reason is worth understanding before you plan around it.

What the existing hole usually is

The standard basement window in a Calgary house built before roughly the last two decades is a wide, short slot set high in the wall. It exists to put a bit of daylight into a utility space. It was never intended as a way out of a building, and it was not sized as one.

Two things follow from that shape.

First, it is short. The minimum dimension requirement — neither the height nor the width of the clear opening may be less than 380 mm (15”) — bites on the height of a slot window, and it bites regardless of how wide the slot is. You can widen a short window across the entire wall and the height has not moved.

Second, it is high. The 1.5 m maximum sill height above finished floor is measured to the sill, and a window installed for daylight tends to sit up under the floor system where the daylight is. Widening does not lower a sill either.

So the two most common reasons an existing basement window is not egress are both immune to making it wider. That is the whole argument in two sentences.

Which means the cut goes down, not sideways

Once you accept that, the shape of the job changes.

Making the opening compliant almost always means removing wall from below the existing window — dropping the sill, extending the opening downward toward the slab. Which is a full-height cut through sound concrete on both jambs, plus the work of dealing with whatever lintel or bearing arrangement already exists over the old opening.

At that point you are cutting concrete on three sides of a rectangle either way. The saving from “reusing” the existing hole has collapsed to the cost of the material that was already missing, which is a small fraction of the job.

Meanwhile you have inherited constraints. The existing opening is where the original builder put it, which may not be where the room wants a window, may not be where the grade outside is most convenient, and may not be the best place structurally.

When enlarging genuinely is the better route

It is not always the wrong answer. Three cases where it clearly wins:

The existing opening is already tall enough and only narrow. Uncommon in older stock, but real — some houses have a reasonably proportioned basement window that just falls short on area. Widening a window that already satisfies the height dimension and the sill limit is a modest job.

The sill is already low enough. Newer houses, and particularly houses built with a future suite in mind, sometimes have windows set low with a decent existing well. If the sill measurement works against the finished floor you plan to build, you are only solving for area.

There is an existing well worth keeping. A properly built, properly drained well of adequate size represents real money in the ground. If the opening can be enlarged within it and the 760 mm clear projection still survives with the sash open, keeping it is a genuine saving.

Note what those three cases have in common: they are all determined by measurement, not by preference. Whether enlarging is cheaper is a question with an answer, and the answer is found by measuring the existing sill against the finished floor and the existing clear opening against 0.35 m² (3.77 sq ft).

What a new opening buys you

The case for cutting fresh, when the existing window is a lost cause anyway:

You put the window where the room needs it. In a basement bedroom that is being laid out from scratch, being able to place the egress opening in the wall that suits the furniture and the door swing is worth something.

You put it where the grade suits it. Grade around a Calgary house is rarely uniform. Moving an opening a few metres along a wall can mean a shallower well, which can mean staying under the 1.2 m depth that triggers the permanent ladder or steps requirement, which is a real simplification of the well.

You put it where the structure suits it. Away from a beam pocket, away from a post above, away from the concentrated load that would have driven the lintel design. The engineer will thank you and the lintel will be smaller.

The thing that decides it is a tape measure

Here is the sequence that gets to a real answer, and it takes about ten minutes in the basement.

1. Measure the clear opening of the existing window — the hole you could climb through with the sash fully open, through the frame. Not the rough opening, not the glass.
2. Measure from the sill down to the floor that will exist when the basement is finished, including any subfloor or sleeper system you plan to build.
3. Look at what is directly above the window: joists running parallel or perpendicular, any post, beam or chimney landing nearby.
4. Go outside and look at the grade at that wall, and at two or three other candidate walls.

If step one and step two both come back comfortably, enlarging is probably your answer. If either fails, assume you are cutting, and the only question left is where.

What we do with that

We measure all four on the site visit and tell you which job you are actually buying, including when the answer is that the existing window is fine and you do not need us. The installation service page sets out the work either way, and the size requirements article explains why the window unit you choose changes the clear opening for the same hole.

For what separates one quote from another once the scope is settled, see the cost page. Or just send us the address.