The Five-Centimetre Problem: Why Your New Window Shrinks the Space for Blinds

Most window quotes are written around a hole in the wall. That is fair enough — the frame has to go somewhere — but it is not the whole story. The bit you actually live with is the opening on the inside: the reveal, the sill, the plaster line and the few centimetres between the glass and whatever you plan to hang in front of it. Get those details wrong and the frame can be perfectly fitted while the room still feels like a compromise.

This is not a scare story about rogue traders. It is a practical warning about scope. A quote can cover a new window and still leave you paying for plaster repairs, a deeper internal sill, a new blind or a moved curtain track. None of those are outrageous costs on their own. Together, they can take the shine off a keen price.

The frame moves; the reveal changes

An old window sits in a wall at a particular depth. The new one may sit further forward, further back, or in roughly the same plane. That choice affects how much internal reveal you have left. Push the frame towards the outside face and you gain internal sill depth — useful for plants, blinds and not knocking your knuckles. Push it the other way and you may improve the external appearance or suit the cavity, but the inside can end up shallower.

Add a thicker frame, a deeper sash profile or a different sill arrangement and the finished opening is not the same as the one you measured with a tape before the job. The glass line can move by 30mm, 50mm, sometimes more. That is the five-centimetre problem: not a disaster, but enough to make a recessed blind stop fitting or a curtain pole foul the new handle.

What actually needs clearance

Blinds are the first casualty. A recess-fit roller or venetian needs a flat headrail area, side clearance and enough depth so the slats or fabric do not sit against the glass or the handle. A face-fit blind needs a flat face to screw into — and new frames often have chamfers, vents and handles exactly where you wanted the brackets.

Curtains are the second. A pole or track needs wall space above the window, plus projection so the fabric clears the sill and any radiator. If the new window sits proud of the plaster line, or the reveal is tighter than before, the old track position may no longer work. Nobody wants to discover that after the decorating is done.

Then there are the everyday collisions: tilt-and-turn handles that swing into the blind, trickle vents that sit where the headrail wanted to go, and radiators that suddenly look much closer to the new sill. These are not exotic problems. They are the ordinary consequences of changing the window without checking the finished opening.

Measure the opening you will actually have

Before signing, measure more than the width and height of the existing frame. Use a tape and a notepad, and take several readings because old openings are rarely square.

• Reveal depth: measure from the internal wall face to the glass line at the top, middle and bottom. Do the same on both sides.
• Sill projection: note how far the current internal sill sticks out from the wall and how much of that you need to keep.
• Plaster line: check whether the wall is flat or whether the plaster returns into the reveal. A lumpy edge can eat clearance.
• Handle swing: open the existing window and watch how far the handle moves. If you are changing to a tilt-and-turn, allow for a bigger sweep.
• Blind or curtain position: measure the recess where a blind would sit, the wall above the window, and the distance from the window to any radiator.
• Vent position: note where trickle vents are now and where they would sit on the new frame.

Take photos with a ruler or tape visible. They are surprisingly useful when the surveyor visits and when you are comparing quotes.

Ask the surveyor these questions

A good surveyor will not be offended by a checklist. They will be glad you have thought about the finished room rather than just the frame colour.

1. How far will the new frame sit from the internal wall face?
2. What will the finished internal reveal depth be, including any new sill or trim?
3. Is an internal sill included, and how far will it project?
4. Who is making good the plaster, and how far does that work go?
5. What clearance will I have for a recess-fit blind or a curtain track?
6. Will the handles or trickle vents interfere with my planned blind position?
7. If the reveal is uneven, what is the tolerance before extra work is needed?
8. Are radiator moves, blind alterations or curtain track changes included — or explicitly excluded?

Get the answers in writing on the quote or survey sheet. “We’ll make it good” is not a measurement. “Frame set 120mm back from internal face, new 150mm sill supplied, plaster patched to reveal” is.

The quote should cover the finished opening

A window quote is not just a price for plastic and glass. It is a price for a finished opening that works with the room. That means the surveyor should be thinking about the reveal, the sill, the plaster, the handle swing and the blind or curtain clearance before the order goes to the factory.

If those things are outside the quote, fine — but they should be named as exclusions. You can then budget for them properly instead of discovering them when the fitter is packing up. A cheap quote that leaves you paying a plasterer, a blind company and a curtain fitter is not quite the bargain it first appeared to be.

Measure the hole, yes. But measure the space around it too. The handle is the least of it; the reveal is where the finished job either settles into the room or starts a second round of spending.