Picture the scene. The scaffold is up, the plasterer is booked for the following week, and the replacement windows arrive on site. Then building control asks where the background ventilation is. The original frames had trickle vents; the new ones don’t. Nobody specified them, so nobody supplied them, and a programme that looked tidy on paper slips by several weeks.

Window replacement on refurbishment work rarely goes wrong at the joinery stage. It goes wrong in the decisions made, or skipped, before the order is placed. On period properties in particular, timber windows raise questions that a standard like-for-like swap never does: planning sensitivity, species and finish, glazing bar detailing, and a lead time longer than many programmes allow for. Settle those early and the installation becomes the straightforward part of the job.

Start with what the building, and the rules, will allow

Before anyone measures an opening, establish the planning position. Replacing windows on a house is often permitted development, but conservation areas complicate that. An Article 4 direction can remove the right to alter windows on elevations facing a highway, and some councils apply those directions across entire streets. On a listed building, replacement almost always needs listed building consent, and conservation officers will expect a convincing case for replacing rather than repairing historic joinery. A short call to the local planning authority costs nothing. Discovering a restriction after the windows have been made costs the whole order.

Building Regulations come next. In England, Approved Document L sets a limiting U-value of 1.4 W/m²K for replacement windows in existing dwellings. Scotland and Wales have their own standards, so check which regime applies before quoting performance figures to a client.

Approved Document F, revised alongside Part L in June 2022, is the one that catches people out. Where the existing windows had trickle vents, the replacements need background ventilators at least as large as the originals. Where they didn’t, the work must not leave the room less ventilated than before, which in practice usually means adding vents to the new frames: the guidance gives 8,000mm² equivalent area for habitable rooms and kitchens and 4,000mm² for bathrooms. A night-latch position on the handle doesn’t count.

Treat timber, glazing and finish as one decision

Specifications often go wrong when species, glazing and finish are chosen separately. They interact, and the right combination depends on how the window will be finished and how exposed the elevation is.

Glazing follows from the frame and the brief. A double-glazed unit with a low-emissivity coating and argon fill is the standard route to meeting the 1.4 W/m²K figure in a timber frame. Triple glazing earns its place on exposed sites and on deep-retrofit projects chasing lower heat loss.

Glazing bars deserve their own line on the schedule. Astragal bars bonded to the face of a sealed unit give the look of divided panes at lower cost; solid timber bars dividing individual panes are more authentic and are sometimes required on listed buildings. Confirm which one the planning consent assumes before ordering.

Finish is the final piece. Factory-applied coatings arrive protected and consistent, cut site painting out of the programme and are applied in conditions a scaffold in November can’t match. Primed-only frames give the decorator control over the final colour but add labour and weather risk. If you are pricing several options, it helps to work from a supplier whose specification options are laid out in one place. Wooden Windows Online, for example, supplies made-to-measure casement and sash windows in engineered pine, meranti and oak, with glazing from single to triple and a choice of finishes, which makes like-for-like comparison against a window schedule much quicker.

Survey properly, because period openings are rarely square

Older buildings settle. Openings drift out of square, sills slope, and the left-hand window in a pair is seldom exactly the size of the right. Measure each opening individually, taking three widths and three heights, and work from the smallest. Note the measuring basis, brick to brick or inside an existing frame, and record any check reveals, where the window sits behind a rebate in the outer brickwork, because they change how the new frame must be sized and fixed.

Two further checks belong on the survey sheet. First, means of escape: where an existing first-floor window serves as an escape route, the replacement should offer an opening at least as large. Approved Document B gives a minimum unobstructed openable area of 0.33m², with at least 450mm in both height and width.

Build the lead time into the programme

Bespoke timber windows are not a stock item. Production typically runs to several weeks once sizes and specification are confirmed, and that clock only starts when the order is signed off and any deposit is paid. On a refurbishment programme, the sequence that works is simple: survey, confirm the schedule with the client, place the order, then plan strip-out and wet trades around the delivery date rather than hoping the windows will catch up.

Deliveries need somewhere to go. Store frames flat or upright as the supplier advises, off the ground, under cover and away from wet trades. Factory-finished timber tolerates handling, but it doesn’t tolerate a week propped against a damp wall beside a cement mixer.

Hand over a window schedule, not just the windows

The most useful document you can leave with a client is a single schedule recording every opening: its size, species, glazing specification, U-value, trickle vent area and finish. Add the coating manufacturer’s recommended inspection and recoat intervals. It answers building control queries, supports any future sale of the property, and tells whoever maintains the windows in ten years exactly what they are working with.

Start that schedule at survey stage and it becomes your ordering checklist as well. Fill it in opening by opening, confirm it with the client, and most of the problems described above never reach site.