Standard Window Sizes in the UK: A Complete Guide by Era and Property Type

November 4 2025

When renovating, restoring, or building a home, understanding standard window sizes is essential—whether you’re replacing sash windows in a Victorian terrace or fitting UPVC units in a modern new-build.

Theoretical standardization encounters multiple challenges during its transition to practical application which creates difficulties for its successful implementation. Window dimensions have evolved through different historical periods because architects adopted various design approaches while building regulations and manufacturing processes advanced.

The guide analyzes window dimensions from Georgian British homes to post-war domestic architecture and modern residential construction in the United Kingdom.

The dimensions of sash windows and casement windows and UPVC windows will be explained to help you determine the right size for your home.

Why “Standard” Window Sizes Aren’t Always Standard

Before diving into measurements, it’s important to recognise that true standardisation in UK window sizes is relatively recent. Historic properties were often built with bespoke windows, crafted on-site or by local joiners. The same street contains different variations of buildings.

The market provides manufacturers with standard metric dimensions which mostly apply to UPVC and contemporary timber frames, yet these dimensions do not fit with older buildings unless modifications are made.

Six Over Six Windows specializes in heritage window solutions which produce custom double-glazed units that preserve authentic dimensions while delivering contemporary thermal insulation and soundproofing performance.

Standard Window Sizes by Property Era

Georgian (1714–1830)

Georgian architecture implemented rigorous design rules that focused on creating harmony through equal proportions and elegant visual features. The rectangular grid pattern of six-over-six sash windows served as the standard for small-pane window organization.

Common window heights: 1.2 m to 2.1 m
Common widths: 0.9 m to 1.8 m (often in multiples of 0.3 m)
Sash box frame depth: ~180–220 mm

The windows on the upper floors of Georgian houses were proportioned to stand taller than they were wide. The windows on the ground floor might extend all the way to the building’s floor in townhouses.

Typical Georgian Sash Window Sizes (mm)

Ground	1200–1800	1800–2100	6-over-6 or 8-over-8
First	1200–1500	1500–1800	6-over-6
Upper	900–1200	1200–1500	4-over-4 or 6-over-6

Note: Builders and regional differences led to variations in exact window dimensions. London townhouses often featured larger windows than rural cottages.

Victorian (1837–1901)

The Victorian era introduced ornate interior decorations along with larger glass windows because glass production advancements took place. Sash windows maintained their popularity but now they used two large panes per sash (e.g., two-over-two design).

Common widths: 1.0 m to 1.8 m
Common heights: 1.3 m to 2.0 m
Sill height: Typically 900 mm above floor

Bay windows gained widespread popularity especially in semi-detached and terraced houses.

Typical Victorian Sash Window Sizes (mm)

Standard sash	1200	1500	Most common in terraces
Bay window unit	2400–3600	1500–1800	Composed of 3–5 sashes
Top-floor sash	900	1200	Smaller, simpler design

Victorian builders often used imperial measurements, so you’ll frequently encounter sizes like 4ft x 5ft (1219 mm x 1524 mm).

Edwardian (1901–1914)

The Edwardian design combined Victorian decorative elements with Arts and Crafts simplicity to create its distinctive style. The windows grew in size while the main entrance received tall sash or casement windows installed on its sides.

Common widths: 1.2 m to 2.0 m
Common heights: 1.5 m to 2.1 m
Increased use of casements, especially in suburban villas

Typical Edwardian Window Sizes (mm)

Front sash	1500	1800	Two-over-two
Side casement	900	1800	Full-height, hinged
Rear casement	1200	1200	Square, practical

Post-War (1945–1970s)

The United Kingdom entered a housing crisis following its involvement in World War II. The necessity for quick construction along with cost savings led to the adoption of standard steel and timber windows for council estates and semi-detached houses.

Metrication began, but many sizes still followed imperial logic
Smaller windows to reduce heat loss and cost
Rise of aluminium and early UPVC in the 1960s–70s

Common Post-War Window Sizes (mm)

1940s–50s	Steel casement	900–1200	1200–1500	Steel
1960s	Timber sash	1000	1300	Softwood
1970s	Early UPVC	1200	1200	PVCu (basic)

These windows often feel “boxy” compared to pre-war designs, with less emphasis on aesthetics.

Modern New-Builds (1980s–Present)

The UPVC material gained supremacy throughout the 1980s while metric measurements became the standard for window size production. The Building Regulations Part L requires energy efficiency standards to determine the dimensions of glazing areas and their thermal performance values.

Common standard UPVC window sizes:

600 mm x 900 mm
900 mm x 1200 mm
1200 mm x 1200 mm
1800 mm x 1200 mm (for patio or French doors)

The manufacturers Anglian and Everest and Safestyle provide standard sizes, but they also offer customized alternatives.

Standard UPVC Casement Window Sizes (mm)

Single casement	600	900	Side-hung
Double casement	1200	1200	Two side-hung panes
Top-hung	900	600	Hinged at top
Tilt-and-turn	800–1000	1200–1500	Dual operation

The manufacturer specifications should be checked at all times because different brands produce sizes which may differ from one another.

Standard Sizes by Window Type

Sash Windows

Sash windows serve as a distinguishing architectural feature which defines period homes throughout the United Kingdom. Specific dimensions show up repeatedly throughout the absence of a defined “standard” system:

Widths: 900 mm, 1200 mm, 1500 mm
Heights: 1200 mm, 1500 mm, 1800 mm, 2100 mm
Box frame depth: 180–240 mm (critical for retrofitting double glazing)

At Six Over Six Windows, we replicate original sash proportions using slim-profile double glazing to maintain heritage aesthetics without sacrificing performance.

Common Sash Window Dimensions (mm)

4-over-4	900	1200	Late Georgian
6-over-6	1200	1500	Georgian/Victorian
2-over-2	1500	1800	Victorian/Edwardian
1-over-1	1200–1800	1500–2100	Edwardian+

Casement Windows

Casements dominate post-1930s housing. The windows function through side or top hinges which allow them to swing outward.

Single-light: 600–900 mm wide
Double-light: 1200–1800 mm wide
Fixed + opening combos: e.g., 1800 mm total width with one 600 mm opener

Standard Casement Sizes (mm)

Single side-hung	600–900	900–1500	Bedrooms, bathrooms
Double side-hung	1200–1800	1200–1800	Living rooms
Top-hung	600–1200	450–900	Kitchens, high walls
French casement	1800–2400	2100	Garden access

UPVC Windows

UPVC windows are sold in metric “standard” sizes, but many suppliers now offer made-to-measure services.

Standard increments: Widths and heights in 100 mm or 300 mm steps
Maximum single pane: ~1200 mm x 1500 mm (for structural integrity)

Popular Off-the-Shelf UPVC Sizes (mm)

Standard casement	600	900	Ideal for small rooms
Large casement	1200	1200	Common in new-build living rooms
Bay window kit	2700	1500	Three-section, pre-fabricated
Sliding sash	1200	1500	Mimics traditional sash

Caution: “Standard” UPVC may not fit period properties without significant brickwork modification.

How to Measure Your Existing Windows

When you replace windows in older homes you need to get exact measurements for the process:

Measure the window opening (reveal), not the existing frame.
Take three width measurements (top, middle, bottom) and use the smallest.
Do the same for height (left, centre, right).
Note the depth of the reveal—critical for double-glazed retrofits.
For sash windows, measure sash box depth and travel height.

We provide free site surveys at Six Over Six Windows to guarantee exact measurements and conservation area compliance when needed.

Final Thoughts: Choosing the Right Size for Your Home

The process of determining standard window sizes in the UK requires selecting dimensions which match the architectural period and design features of your home and its functional requirements.

Pre-1930s homes: Prioritise authenticity—custom sash windows are worth the investment.
Standard UPVC materials work well for post-war homes but timber or composite materials provide better visual appeal.
New builds: Take advantage of standard metric sizes, but don’t sacrifice daylight or ventilation for convenience.

The correct window dimensions will boost both visual appeal and operational efficiency for Georgian townhouses in Bath as well as 1960s semi-detached homes located in Birmingham.

No Need to Replace—Just Refurbish and Retrofit

Crucially, if you own a period property with original timber sash windows, full replacement is rarely necessary. At Six Over Six Windows, we specialise in refurbishing your existing windows and retrofitting them with 8 mm Vacuum Insulated Glass (VIG)—a slim, high-performance glazing solution that delivers triple-glazing efficiency without altering your window’s historic appearance or proportions.

The method protects architectural elements and prevents planning difficulties in conservation areas while delivering cost savings of up to 60 percent compared to new window installations. Our full service includes:

Full timber frame restoration (from £350 per window)
Draught-proofing and weight rebalancing
Installation of 8 mm VIG units (from £700 per window)
Complete refurbishment and reglazing packages from £1,150

See our transparent pricing and package options: View sash window refurbishment & double glazing costs