The Ultimate Guide to Choosing Windows, Doors & Rooflights For Extensions and Loft Conversions
Choosing the right windows, doors and rooflights affects daylight, comfort, energy bills, planning approval and building control sign-off.
This guide explains what actually works for UK extensions and loft conversions — without jargon.
Extensions vs Loft Conversions
Extensions
Often rely on large patio doors and roof glazing
Overheating risk is higher due to bigger glass areas
Loft conversions
Limited wall space
Depend heavily on sloped roof windows
Glazing position affects head height, privacy and comfort
Glazing Performance Explained
U-value (heat loss)
Lower is better.
Modern double glazing: 1.2–1.4 W/m²K
Triple glazing: 0.8–1.0 W/m²K
G-value (solar heat gain)
Controls overheating.
High g-value = more solar heat
Low g-value = more heat reflected (important for roof glazing)
Window Types
Casement windows
Cost-effective
Good ventilation
£350–£1,500 installed
Tilt & turn windows
Better airtightness
Constricted tilt opening good for upper floors or limited space
£700–£2,000+
Sash windows
Planning-friendly for heritage homes
Portrait format restricts views
£600–£2,500+
Sloped Roof Windows
Main daylight source in loft conversions
Very high solar gain → solar-control glazing recommended
Pros
✔ Excellent daylight
✔ Natural ventilation
✔ Usually planning-friendly
Cons
✖ Overheating risk if oversized
£600–£1,500+ installed
Flat Rooflights vs Roof Lanterns
Flat rooflights
Clean, minimal appearance
May need more external cleaning
Better controlled daylight
£450–£1,300+
Roof lanterns
Strong visual impact
Less external cleaning required
Highest overheating risk if poorly specified
£1,500–£6,000+
Patio Doors & Large Openings
Sliding doors
Best for wide openings
Fixed panes may block flow
£1,000–£3,500
Bi-fold doors
Fully openable
Panels need space to stack when fully open
£2,500–£6,000+
Lift-and-slide doors
Best thermal performance
Costly option
£4,000–£10,000+
Crittall-style doors
Offer slimmer sightlines and stronger visual impact
Open less fully and usually cost more for similar thermal performance
£3,000–£10,000+
Frame Materials
UPVC
✔ Cheapest | ✔ Low maintenance
✖ Bulkier frames
Aluminium
✔ Slim sightlines | ✔ Strong for large openings
✖ Higher cost
Timber
✔ Excellent insulation | ✔ Traditional look
✖ Maintenance required
Composite (timber-aluminium)
✔ Best performance
✖ Most expensive
Planning & Building Regulations
Planning
Glazing/frame scale and proportion matter
Extra care in conservation areas
Building Regulations
Part L: heat loss (U-values)
Net proposed glazing area more than 25% of new floor area requires SAP calculation
Part O: overheating risk
Solutions include solar-control glazing, controlled sizes and opening roof windows.
Common Mistakes to Avoid
Oversized rooflights
Choosing on looks alone
Ignoring orientation
Leaving glazing decisions too late
How Draw & Plan Helps
At Draw & Plan, we:
design glazing layouts that pass planning & Building Control
specify compliant U-values and g-values
reduce overheating risk early
avoid costly late-stage changes
Final Advice
Good glazing should:
feel comfortable year-round
meet regulations first time
suit your home’s character
be designed properly from day one
Disclaimer: This article is for general informational purposes only and does not constitute legal or professional advice. Always consult relevant professionals and local authorities before undertaking any development or change of use.
