Working with an Architect on Your Surrey Extension: How Window Specification Works

If your architect has specified replacement windows and doors as part of your Surrey extension, the drawing set they have handed you is the start of a conversation, not the end of one. Somewhere between that drawing and the finished installation, a set of performance figures on paper has to become a specific product, ordered to a specific opening, fitted by someone who was not in the room when your architect drew it.

That handover is where most of your anxiety about a renovation project probably sits. You worry the installer will not read the drawings properly, or will not sequence the work around the rest of your build. Fair worry. We have watched it go wrong for other people's projects before we ever got involved. This article explains how the handover actually works: how we read a glazing schedule, where your architect's responsibility ends and our survey begins, and how a performance specification becomes an actual manufacturer choice.

Reading a Glazing Schedule

On an architect-led extension, replacement windows and doors are rarely specified in isolation. Your glazing schedule is usually a table, keyed to the drawings, that lists every window and door opening in the project. Against each opening it records a reference number, a rough dimension, a glazing specification, and often a performance target such as a U-value, an acoustic reduction figure, or a fire or escape rating where the room requires one. Straightforward, on paper.

What the schedule deliberately does not confirm is the finished reality of the opening. It cannot, because the opening has not been built yet, or has not been surveyed once it has. Under the RIBA Plan of Work 2020 overview of Stage 3 and Stage 4, an outline specification is set at Stage 3, once the design is spatially coordinated, and the full technical design, including final product selections, is developed at Stage 4. A glazing schedule handed to an installer usually sits somewhere in that gap: firm enough to work from, not yet confirmed against the actual building.

We read a glazing schedule the same way we read a structural drawing: as the intent, not the finished order. The biggest risk in a renovation-context window project is sequencing, not product selection, and that starts with treating the schedule as a brief to survey against, not a list to price up and confirm on the spot.

Where the Architect's Drawing Ends and Our Survey Begins

The architect's drawing sets design intent: the position of each opening, its approximate size, and the performance it needs to achieve. What it cannot set is the as-built reality, because that only exists once the structural work is underway. That is what a site survey is for.

Our survey confirms the openings as built, not as drawn: the exact dimensions, the structural tolerances, the condition of the lintel and reveal, and anything the drawing could not have anticipated. This is also the point at which FENSA registration matters. Building control sign-off for the extension's structure and the certification of the window and door work are two separate things. FENSA's explanation of how installer registration meets building regulations confirms that FENSA is the government-authorised scheme that monitors building regulation compliance for replacement windows and doors, which means the window and door installation is notified and certified without a separate council application running alongside the wider extension's building control process.

Our team's background running Fleet Homes Construction means we read a structural programme, not just an order for replacement windows and doors. That matters more on an architect-led job than on a straightforward replacement, because the windows are one line in a much longer sequence, and a company that only understands windows will not know when to wait.

How a Performance Specification Becomes a Manufacturer Choice

A figure on a glazing schedule is not itself a product. A U-value, an acoustic target, or an escape dimension is a requirement that has to be matched against what named manufacturers of replacement windows and doors actually build, and that is where a manufacturer-agnostic installer earns their place.

Take thermal performance first. Under Approved Document L's Table 4.2 standard for windows in existing dwellings, a replacement window must not exceed a U-value of 1.4 W/(m²K), or meet a minimum Window Energy Rating of Band B, and the replacement's performance must be no worse than the window it replaces. That figure has to be matched against real product data sheets across uPVC casement windows, aluminium, and composite ranges, not assumed from a brochure. Acoustic targets work differently again, and are met through the glass makeup rather than the frame: an architect specifying an extension near a busier road or under a flight path will sometimes set a target decibel reduction, and in most cases that is achieved with asymmetric double glazing or a laminated pane, which is why the specification needs translating into glass build-up, not just a product name.

Fire and escape requirements are the most concrete example. On a habitable room in an extension, Approved Document B's escape window dimensions for habitable rooms require an unobstructed opening of at least 0.33m², no dimension under 450mm, and a cill no higher than 1100mm from the floor. That figure affects which casement configuration is buildable in the opening the architect has drawn, not just which product line gets specified.

We are not tied to a single manufacturer, so a specified performance figure gets shopped against several ranges of replacement windows and doors rather than fitted around whatever one supplier happens to stock. Once the target is set, the job is finding the product that actually meets it in our windows range, not steering the specification toward a single system because that is what we sell.

Briefing Both Sides So They Coordinate Cleanly

Briefing both sides properly is what separates a smooth order for replacement windows and doors from one that stalls at survey stage. The cleanest architect-led projects share one habit: both sides know which figures on the schedule are fixed and which are placeholders waiting on a survey. Ask your architect which is which before the project moves further. Ask your installer whether the survey happens before or after structural sign-off, and what happens to the order if the opening changes afterwards.

We will be honest about the cost of doing this properly: surveying against a schedule rather than pricing straight off it takes longer at the front end, sometimes a couple of weeks longer than a homeowner expects when they first ask for a number. We would rather lose a bit of speed at quote stage than hand you a price built on openings that do not yet exist.

This coordination is not theoretical for us. On a kitchen and dining extension on a 1600s property in Hindhead, designed by C7 Architects in Guildford, and on a double and single-storey extension shell for a period cottage in Seale, designed by Michael Conoly Associates in Farnham, the window specification had to be surveyed and confirmed against a live structural programme, not against a finished building. The same applied on an aluminium windows and bifold doors project in Haslemere, where the openings were part of a renovation still in progress rather than a settled shell.

We default to surveying before ordering on every architect-led project, even when the schedule looks firm, because a confirmed order against an opening that later shifts costs more time than the survey ever does. If your project involves a window relocation as part of a wider layout change, the same sequencing logic applies to our window relocation service, where the new opening has to be confirmed structurally before a frame is cut for it.

What we would recommend to anyone running an architect-led extension in Surrey is simple: treat the glazing schedule as the brief, not the order, and choose an installer who will say so before you spend money on a product that does not yet have a confirmed home to go into.