The Ultimate Guide to Choosing Windows, Doors & Rooflights For Extensions and Loft Conversions
Choosing the right windows, doors and rooflights is one of the most important design decisions you’ll make when extending or converting your home.
They affect:
natural light
heat loss and overheating
energy bills
planning approval
building control sign-off
how the space actually feels to live in
This guide explains everything you need to know — from window types and frame materials to glazing performance, costs, and UK regulations — so you can make confident, future-proof choices.
1. Start With the Type of Project
Before choosing products, you need to understand how extensions and loft conversions behave differently.
Extensions
Often rely on large glazed doors to the garden
Roof glazing is common where side light is limited
Overheating is a growing issue due to large areas of glass
Loft Conversions
Limited wall space for windows
Heavy reliance on sloped (pitched) roof windows
Glazing placement is critical for head height, privacy and daylight
2. Understanding Glazing Performance (In Plain English)
U-Value — Heat Loss
Measures how much heat escapes through glass
Lower = better insulation
Required by Building Regulations Part L
Typical values
Old glazing: ~2.8
Modern double glazing: 1.2–1.4
Triple glazing: 0.8–1.0
G-Value — Solar Heat Gain
Measures how much solar heat enters the room
Scale from 0.0 to 1.0
Lower values = more heat reflected
Why this matters
High g-values can cause summer overheating
Especially important for:
rooflights
south-facing glazing
large sliding doors
Low-E & Solar-Control Glass
Most modern glazing includes:
Low-E coatings to reflect internal heat back inside
Solar-control coatings to reduce overheating
These coatings are invisible but critical for comfort.
3. Window Types for Extensions & Lofts
Casement Windows
Best for: Most extensions and traditional homes
Hinged side-opening windows
Easy ventilation and good thermal performance
Costs (installed)
UPVC: £350–£600
Timber: £700–£1,200
Aluminium: £900–£1,500
✔ Planning-friendly
✔ Cost-effective
Tilt & Turn Windows
Best for: Modern extensions and upper floors
Tilt for ventilation, turn for cleaning
Excellent airtightness
Costs
UPVC: £700–£1,100
Aluminium: £1,200–£2,000+
✔ Secure and flexible
✖ Slightly higher cost
4. Sloped (Pitched) Roof Windows — Essential for Lofts
Used in:
loft conversions
pitched-roof extensions
Key Features
Installed flush with roof slope
Centre-pivot or top-hung options
Often the primary daylight source in lofts
Thermal & solar performance
U-values: ~1.1–1.3
Very high solar gain → solar-control glass recommended
Costs (installed)
Fixed: £600–£1,000
Opening: £900–£1,500+
Pros
✔ Excellent daylight
✔ Natural ventilation (stack effect)
✔ Usually easier for planning approval
Cons
✖ Overheating risk if oversized
✖ Must be positioned carefully for glare and privacy
5. Flat Rooflights & Roof Lanterns (Extensions)
Flat Rooflights
Best for: Flat-roof rear extensions
Minimal external appearance
Strong daylight penetration
Costs
Fixed: £450–£900
Opening: £800–£1,300+
✔ Clean, modern look
✖ Oversizing can cause overheating
Roof Lanterns
Best for: Statement kitchen or living spaces
Large glazed structures
Dramatic light levels
Costs
Small: £1,500–£2,500
Large: £3,000–£6,000+
✔ Visual impact
✖ Most common source of overheating complaints
6. Patio Doors & Large Openings
Sliding Doors
Ideal for wide openings
Slim aluminium frames available
Costs
UPVC: £1,000–£1,800
Aluminium: £2,000–£3,500
Bi-Fold Doors
Fully openable
Strong indoor-outdoor connection
Costs
Aluminium (3–5 panels): £2,500–£6,000+
✖ More frame = slightly worse thermal performance
Lift-and-Slide Doors
Heavy panels, excellent seals
Best thermal performance for large spans
Costs
£4,000–£10,000+
✔ Premium performance
✖ Higher upfront cost
7. Frame Materials — What Should You Choose?
UPVC
Pros
Cheapest
Low maintenance
Good insulation
Cons
Bulkier frames
Less premium appearance
Best for: Budget-conscious projects
Aluminium
Pros
Slim sightlines
Strong for large openings
Modern aesthetic
Cons
Higher cost
Must include thermal breaks
Best for: Sliding & bi-fold doors
Timber
Pros
Excellent insulation
Traditional appearance
Cons
Requires maintenance
Best for: Period homes and conservation areas
Composite (Timber-Aluminium)
Pros
Excellent thermal performance
Low maintenance
Premium finish
Cons
Most expensive
8. Planning Permission Considerations
You may need planning permission if:
glazing significantly alters the appearance
works are visible from the street
property is in a conservation area
rooflights protrude excessively
Good drawings reduce delays and objections.
9. Building Regulations & Overheating (Part L & Part O)
Modern regulations require:
thermal compliance (Part L)
overheating risk assessment (Part O)
Solutions include:
solar-control glazing
controlled g-values
opening rooflights
shading strategies
Glazing must be designed into the drawings, not chosen at the last minute.
10. Common Homeowner Mistakes
❌ Choosing glazing on appearance alone
❌ Oversizing rooflights
❌ Ignoring solar gain
❌ Mixing frame styles poorly
❌ Leaving window decisions until after planning
How Draw & Plan Helps
At Draw & Plan, we:
design glazing layouts that meet planning & Building Regulations
specify appropriate U-values and g-values
reduce overheating risk early
coordinate structural openings
avoid costly late-stage changes
Final Advice
The best windows, doors and rooflights:
feel comfortable year-round
meet regulations without stress
look right for your home
are designed properly from the start
If you want your extension or loft conversion to look great, perform well and pass approval smoothly, glazing decisions shouldn’t be left to chance.
