Specifying timber windows for a new build or extension in London is a different exercise from replacing windows in a period property. There’s no conservation officer to satisfy, but there is Building Regulations Part L to meet, a design team to coordinate with, and performance figures that need to stack up on paper before a single frame is made. Here’s what architects, self-builders and developers actually need to know.

Building Regulations: What Part L Actually Requires

Part L of the Building Regulations sets minimum energy efficiency standards for new windows and doors in England. For new-build dwellings and most extensions, this means a maximum U-value requirement — currently 1.4 W/m²K for replacement windows in existing dwellings, and tighter whole-dwelling targets for new builds assessed via SAP calculations. A well-specified timber window with modern double or triple glazing comfortably meets these figures — timber’s natural insulating properties mean the frame itself contributes less thermal bridging than aluminium, and performs close to uPVC without the plastic.

For new-build projects, windows are one input into the wider SAP (Standard Assessment Procedure) calculation for the whole dwelling, so U-values, glazing ratios and orientation all interact — this is worth agreeing with your architect or SAP assessor before frames are ordered, not after.

Choosing Between Sash and Casement for a New Build

Unlike a period property, a new build isn’t constrained by matching existing joinery, which opens up the choice properly:

• Casement windows generally achieve better airtightness and U-values than sash windows because the sealing mechanism is simpler — a strong option where energy performance is the priority.
• Sash windows remain popular on new builds designed in a traditional London style (common in conservation-adjacent infill developments), and modern spring-balanced sash mechanisms with double glazing now perform close to casement equivalents.
• Both can be built to match any elevation design, glazing bar pattern, and paint finish specified by the architect.

Coordinating with the Design Team

The details that most often cause delays on new-build projects aren’t the windows themselves — they’re the interfaces: how the frame meets the cavity wall, cill detailing, and how window openings are sequenced against the build programme. Getting accurate sizes and reveal details confirmed before frames go into production avoids the two most common causes of on-site delay: frames ordered to the wrong opening size, and window fitting scheduled before the structural opening is actually ready.

For extensions specifically, matching new timber windows to an existing property’s proportions and glazing bar pattern — even where the style isn’t identical — usually gives a better result than a jarring contrast, both aesthetically and for resale value.

Lead Times and Programme Planning

Bespoke timber windows are made to order, not stocked — realistic lead times run 6-10 weeks from confirmed order and final measurements to delivery, longer for larger or more complex projects. On a new-build or extension programme, this means window specification and ordering needs to happen well before the structural opening is ready, not once it is. Building this into the programme early avoids window fitting becoming the item that holds up practical completion.

Working with International Windows Group on New-Build Projects

We work directly with architects, contractors and self-builders to specify bespoke timber windows that meet Part L requirements while matching the design intent of the project — from full new builds to single-storey extensions. Get in touch with your drawings or specification for a detailed quote and lead time.