Windows for New Build Extensions: What You Need to Know

Specifying windows for a new extension is different from replacing existing windows — the planning and Building Regulations requirements are applied from scratch, and getting the specification wrong at the start costs significantly more to correct later.

What This Guide Covers

• Building Regulations Part L requires minimum Uw of 1.4 W/m²K for replacement and new windows
• Aluminium is now the dominant choice for extension glazing in London — slim lines, low maintenance
• Window-to-wall ratio matters: excessive glazing in an extension can fail Part L thermal compliance
• Conservation areas: material and style requirements can significantly constrain extension glazing
• FENSA registration covers replacement windows only — new extension glazing needs Building Control sign-off

Introduction

London homeowners face unique challenges when it comes to glazing — from period properties with sash windows in conservation areas, to modern developments requiring structural glass installations. Getting the right advice before you commit can save you significant time and money.

At BuilderFix Glazing, we have completed thousands of glazing jobs across Greater London and Surrey — from emergency repairs in Kensington and Chelsea, to bespoke walk-on glass installations in Mayfair and Belgravia. This guide is based on real experience and the questions our customers ask us most often.

What Does This Typically Cost in London?

Costs vary depending on the type and size of work, the glass specification, frame material, and access. As a general guide, always obtain a free written quote before committing — this protects you from unexpected charges.

At BuilderFix Glazing, we provide free, no-obligation written quotes for all work. Our prices are transparent and fixed before we start — no surprises on the invoice.

What to Ask When Getting a Quote

• Is the price fixed, or could it change once work starts?
• What glass specification is included?
• Are all fixings, sealants and finishing included?
• What compliance documentation will I receive?
• What happens if there is an issue after completion?