Windows and doors look like the simplest line on any estimate. There are eight of them, they are on the drawing, you count them and multiply by a price. That confidence lasts until the first quote comes back with a different number for every unit and a separate schedule for the extras.
The reason is that an opening is not a product, it is a specification. Frame material, glazing, thermal rating, opening method, ironmongery, finish, and the way it is fixed into the wall all change the price. Two windows of identical size can differ by a factor of three on specification alone.
So the takeoff has two halves. The first is a schedule of openings with every attribute recorded. The second is the fixing and finishing work around them, which is where builders lose money.
Build a schedule, not a count
A window and door schedule is a table with one row per opening and enough columns to price it:
Reference and room
Type: casement, tilt and turn, sliding, fixed, door, French door
Width and height, overall frame size
Material and colour, inside and outside
Glazing specification: double or triple, coated, laminated, toughened
Opening direction and handing
Ironmongery, restrictors, handles, locks
Sill type and whether it is included
Fire rating, where required
Fixing method and cavity closer
Filling that table takes an hour on a small house and it turns a vague request into a comparable quote. It is also the document that protects both sides when a delivery arrives wrong.
Measure the structural opening, not the glass
There are three dimensions that matter and they are not the same number:
Structural opening: the hole in the wall, brick to brick or stud to stud
Frame size: the outer dimension of the window, which must be smaller than the opening by a fixing gap
Sight size: the visible glass area, which is what the client thinks they are buying
Frame size is typically 10 to 20 mm smaller than the structural opening in each direction, and the gap is taken up by packers, foam, and a sealant or a bead. On a replacement into an existing opening, the surveyor will measure the opening and the new frame will be made to suit.
The estimating point is that glass area, frame size and opening size all appear on the quote in different places, and a client comparing quotes on glass area is comparing something that is not what they are paying for.
Glazing: where the specification hides
A double glazed unit is not a commodity. The price moves with:
Element
Effect on price
|---|---|
Double to triple glazing
25 to 45 percent more
Low emissivity coating
Small increase, large performance gain
Argon or krypton fill
Small increase
Laminated glass
15 to 30 percent more
Toughened glass where required
10 to 20 percent more
Acoustic laminate
25 to 40 percent more
Solar control coating
15 to 25 percent more
Safety glazing is the one item that is not optional and is often missed. Any glazing below 800 mm from the floor, in a door, or in a door side panel must generally be safety glass. On a schedule of ten windows, two or three will usually fall into that category, and the difference in price per unit is real.
Doors are two products, not one
A door is a leaf plus a frame plus ironmongery plus a wall opening, and internal and external doors have almost nothing in common.
Internal doors: leaf, lining, architrave, hinges, latch, handles. Counted as units, with the linings and architraves measured in linear metres. A standard lining is a set; architrave is 5.5 m per door on both sides.
External doors: a door set with a threshold, a weather seal, a locking system, and usually a thermal rating. A composite or aluminium entrance door costs several times an internal leaf, and it needs a correctly drained threshold detail.
Fire doors: a certified door set with intumescent seals, a closer, and often a specified frame. A fire door is not a door with a sticker, and substituting a standard set is a compliance failure that a building inspector will find.
The fixing and finishing work nobody quotes
The window supplier quotes a window. The builder has to put it in, and that work is a set of lines that appear on no window quote:
Removing the old unit and disposing of it
Making good the reveal, inside and out
Insulation and a cavity closer at the perimeter
Expanding foam, tape or a membrane to the gap
Silicone or a bead externally and internally
Sill, either a new one or a repair of the old
Plaster or render patching around the reveal
Painting or decoration to the make good
Scaffolding or a tower to reach upper openings
On a full replacement of eight windows in a rendered house, the making good and decoration can easily be 40 percent of the installed cost. A client comparing a window quote with an installed price is comparing a product with a project.
A worked example
Six windows and two doors on a two storey house, PVC-U casements, replacing existing in the same openings:
Windows: four at 1200 x 1350, two at 900 x 1200. Frames supplied at opening size less 10 mm
One window has safety glazing to the lower panel because the sill is 700 mm from the floor
Doors: one composite entrance set, one aluminium patio slider
Two windows are at first floor, so scaffold or a tower for two elevations
Installation: remove, fit, pack, foam, seal, and make good inside
Making good: two windows in render, four in plaster, plus internal reveals
Decoration: two coats to all reveals and patching
The windows themselves might be the smaller half of the total. The scaffold, the removal, the render repairs and the decorating are the rest, and they belong in the same estimate even if they sit in different trades.
Thermal performance and the compliance trap
Replacement glazing usually has to meet a thermal performance standard, expressed as a U value or a window energy rating. The requirement varies by country and by whether the work is a replacement, an extension or new build.
Two practical consequences. The specified glazing may be more expensive than the cheapest compliant unit, so a quote built on the lowest specification may fail inspection. And whole house calculations, where a designer has traded glazing performance against wall insulation, mean changing one window can affect the compliance of the whole dwelling.
An estimator who ignores the compliance requirement produces a competitive price that cannot be built. It is worth asking for the specification number before pricing.
Productivity, and why fitting is slower than it looks
A two person team fits 4 to 6 windows a day in existing openings with the making good done as they go, and 8 to 12 in new build where the openings are prepared and the reveal work is left to the plasterer.
The difference is entirely in the existing fabric. An old opening is never square, the old frame is often fixed with something unexpected, and the reveal may be damaged once the old unit comes out. On older properties, allow half a day per window rather than the new build rate, and say so in the price.
Blinds, curtains and the reveal you forgot
A window is not finished when the frame is in. The reveal depth decides whether a blind fits inside the recess, whether a curtain can hang beside the frame, and whether a radiator still has space.
On a replacement where the new frame is thicker than the old one, the reveal becomes shallower. A blind that fitted before no longer does, and a client who was not warned has a legitimate complaint about a window that has technically worked perfectly.
Worth mentioning at survey and quoting as an option: integrated blinds inside the sealed unit, which cost more up front and avoid the problem entirely in a bathroom or a kitchen where the glass is often wet.
Common errors
Counting openings without a specification, so the price is a guess
Forgetting safety glazing where required
No allowance for removal and disposal
Missing the reveal make good on a replacement
No scaffold or access line for upper floors
Architraves and linings not measured separately
Fire doors specified as standard doors
Thresholds and drainage on external doors not detailed
Ordering, lead times and the risk of a late survey
Windows are made to measure and cannot be returned. That combination, made to order plus non returnable, is the reason the survey is the most important appointment on the job.
Two habits protect the estimate. First, the survey happens after the openings are formed and before any plastering, because an opening measured from a drawing is a guess and a guess that arrives wrong is a four week delay and a full replacement. Second, the survey notes the structural opening, the reveal depth, the sill height and the wall construction in one go, so the fixing method and the cavity closer are decided before manufacture rather than on the day of installation.
Lead times of four to eight weeks are normal for made to measure units. On a programme where the windows are on the critical path, that lead time is the programme. Ordering late is not a risk of extra cost, it is a certainty of a delayed handover.
Ventilation, and the detail that gets missed on airtight houses
A modern window has a seal, and a sealed house needs ventilation. Trickle vents, or a whole house ventilation system, are a requirement in many jurisdictions for replacement and new glazing, and they are easy to overlook when a quote is being compared on price per unit.
The cost is modest and the consequence of omitting it is condensation and a failed inspection. On a house that is being made more airtight by other work, glazing replacement is the moment to decide how the building breathes, because the window is where the old house used to leak.
Where the money and the complaints both come from
Two items generate most of the after sales work on glazing: condensation between the panes and problems with the door furniture. The first is a failed seal, usually covered by warranty if it is inside the sealed unit and not at its edge. The second is usually installation or adjustment rather than the product.
Neither is a quantity problem, but both are worth mentioning when you hand over, because a client who understands that a sealed unit can fail in year eight, and that it will be replaced under warranty rather than sold to them again, is a client who does not spend the next five years telling people the windows are faulty.
Frequently asked questions
How do I compare window quotes? By unit, on identical specification: frame material, glazing build up, U value, ironmongery and whether removal and making good are included.
What is the difference between structural opening and frame size? The structural opening is the hole in the wall. The frame is 10 to 20 mm smaller in each direction, with the gap sealed.
Do I need safety glass? In most jurisdictions, yes, for glazing below 800 mm from the floor, in doors and in side panels next to doors.
How long does a window replacement take? Four to six per day for a two person team in existing openings, with making good included.
Are fire doors different from normal doors? Yes. A certified door set with intumescent seals, a closer and a compliant frame. Substituting a standard door fails inspection.
Should I replace frames or just the glass? Replacing the glass only is cheaper and does not fix thermal bridging, draughts or failed hardware. On a house older than 25 years, a full replacement is usually the better value.
Estimate the whole opening in FRELUX
The FRELUX window and door calculator takes a schedule of openings, applies glazing and ironmongery options, and adds removal, making good and access, so the price covers the opening rather than the product alone.