Precision Engineering
Every Glass Box Extension is individually engineered around your architecture, proportions, and how you want to use the space.
Established 1973 • Over 2,000 Projects Completed
Glass Box Extension & Frameless Glass Extension
Minimalist Modern Design, Maximum Light
A bespoke glass box extension, also known as a frameless glass extension, uses ultra-slim structural glazing to connect your home and garden. Floor-to-ceiling glass, contemporary detailing, and New Generation Glass options help create a comfortable year-round living space.
Free Home Design Visit
Up to 10-Year Guarantee
A++ Thermal Rating Achievable
Prices from £40,000
Call 01243 538999. David, our digital assistant, will take a few details so the right specialist can follow up personally.
About Glass Box Extensions
What is a Glass Box Extension?
A glass box extension or frameless glass extension is a contemporary architectural addition built with structural glazing. Minimal framing creates an almost invisible boundary between your living space and garden, maximising natural light and views.
At Room Outside, we specialise in bespoke Glass Box Extensions featuring floor-to-ceiling glass panels, flat structural glass roofs, and large-format sliding or pivot doors. Each installation is precision-engineered to create a seamless indoor-outdoor living experience.
Alongside our Glass Box Extensions, we also design and install bespoke conservatories and orangery extensions across England, including West Sussex, East Sussex, Surrey, Hampshire, Kent, Essex, Greater London, Berkshire, and Dorset.
Maximum natural light
Contemporary aesthetic
Seamless indoor-outdoor living
Large-format sliding doors
Up to 10-year guarantee
Full planning support included
Structural engineering included
More than 50 years of experience
Prices from
£40,000
See the full frameless glass range
Our brochure includes detailed specifications, project photography, and design options for structural and frameless glass extensions.
Download Frameless Glass Brochure
What Sets Us Apart
Why Room Outside?
Four things premium homeowners consistently value when choosing Room Outside for a Glass Box Extension.
New Generation Glass
Advanced glazing options are designed to reduce unwanted solar gain while maintaining natural light and year-round comfort.
Insured Guarantees
Depending on product and location, installations may include up to a 10-year structural guarantee backed by independent UK insurers.
Full Project Management
We manage structural engineering, planning support, building regulations, foundations, installation, and finishing.
Our Work
Glass Box Extension Projects
A selection of Glass Box Extension photography from Room Outside projects.
2026 Price Guide
How Much Does a Glass Box Extension Cost?
A bespoke frameless glass extension from Room Outside starts from £40,000. Larger or more complex structural-glass projects can exceed £100,000, so the final price is confirmed after a design consultation and site survey.
From £40,000
This is the guide starting price for a Room Outside glass box extension. The written quotation confirms the exact design, specification and work included.
£40,000–£100,000+
This is a practical planning range for many bespoke frameless glass extension projects. Size, engineering, access and finish can move the total above it.
Main Cost Factors
Glazing area, roof and door spans, structural steel, foundations, thermal specification, bespoke shapes, site access and internal finishes all affect the final price.
For broader budgets and price-per-square-metre guidance, read our 2026 glass extension cost guide.
Premium Features
Glass Box Extension Features
Every Glass Box Extension is precision-engineered with premium structural glazing and thermal technology options.
Frameless Structural Glazing
Structural safety glass and glass purlins create clean, minimal sightlines with specialist silicone bonding.
Floor-to-Ceiling Glass
Full-height glazing maximises garden views and floods the space with natural light.
New Generation Glass
Advanced glazing helps control heat and glare while supporting year-round comfort.
Sliding and Pivot Doors
Choose large-format sliding, pivot, or bi-fold doors for dramatic openings.
Structural Glass Roofs
Optional structural glass roofs create striking visual connections and excellent daylight.
Structural Engineering
We manage calculations, building regulations, and building control support for every project.
Door Options
Door and Opening Options
Choose from premium door systems to complete your Glass Box Extension.
Sliding Glass Doors
Up to 6m opening width
Ultra-slim profiles
Flush floor tracks
Pivot Doors
Up to 3m height
Hidden pivot mechanism
Dramatic entrance
Bi-Fold Doors
Multiple panel options
Full corner opening
Low threshold
Buyer Guide
Glass Box Extension Comparisons
Both orangeries and conservatories can create stunning extensions, but each suits a different style of home and brief.
Glass Box Extension vs Orangery
- Style
- Contemporary, frameless, minimalist
- Traditional with solid perimeter and lantern
- Glazing
- Structural glazing with near-invisible joints
- Framed glazing with insulated perimeter
- Thermal
- A++ achievable with New Generation Glass
- A++ achievable with enhanced insulation
- Price from
- £40,000
- £35,000
- Build time
- 12 to 16 weeks
- 10 to 14 weeks
- Best for
- Modern homes and maximum glass
- Period homes and a built-in feel
Glass Box Extension vs Conservatory
- Design
- Frameless structural glazing
- Framed glazing with pitched or flat roof
- Aesthetic
- Ultra-contemporary and minimalist
- Traditional or modern depending on style
- Natural light
- Maximum glass on all sides
- Excellent glazed roof and walls
- Thermal
- A++ achievable with New Generation Glass
- A+ achievable with New Generation Glass
- Price from
- £40,000
- £25,000
- Build time
- 12 to 16 weeks
- 8 to 12 weeks
- Best for
- Architectural statement and seamless views
- Light-filled family rooms
Compare related pricing guides: Glass Extension Prices UK 2026, Orangery Prices UK 2026, and Bespoke Conservatories.
Free Consultation
Not sure which glass extension is right for your home?
Book a free home consultation. We will visit, assess structural requirements, and show you what is possible with no obligation.
Our Process
How a Project Works
From first conversation to finished Glass Box Extension, this is what to expect.
Step 1
Free Home Design Visit
A specialist visits your home to discuss your vision, assess structural requirements, take measurements, and discuss glass specification options.
Step 2
Design and Engineering
We produce detailed architectural drawings and structural engineering plans so you can see what the finished extension will look like before committing.
Step 3
Build and Installation
Your Glass Box Extension is precision-built using structural glazing, then professionally installed from foundations through to finishing.
Step 4
Handover and Aftercare
We check the completed project with you and confirm the applicable guarantee and product warranties.
Verified Customer Reviews
What Our Customers Say
Independently verified Google reviews from homeowners sharing their Room Outside experience.
★★★★★
“Room Outside did an amazing job with our glass extension. The design is modern, the build quality is excellent, and the team was professional from start to finish.”Vladimir
★★★★★
“We have always had a problem of our conservatory overheating. The New Generation Glass has made such a difference and it is now a room we can use all the time.”Tom Fordyce
★★★★★
“Very happy with the work Matt did for us with our glass conservatory. They were professional, tidy, and listened to our feedback.”Emmanuelle Dehon
Expert Answers
Glass Box Extension & Frameless Glass Extension FAQs
Common questions about glass box extension prices, frameless glass extension specifications, planning, performance, and installation.
A Glass Box Extension is a minimalist architectural addition using structural glazing. It creates an almost invisible boundary between your living space and garden, maximising natural light and views.
A frameless Glass Box Extension typically costs from £40,000 to £100,000+ depending on size, glass specification, and complexity. Room Outside offers premium Glass Extensions from £40,000 fully installed with building control and up to 10-year guarantee.
The exact scope is confirmed in the written quotation. A typical Room Outside project can include design, structural engineering, foundations, structural glazing, doors, installation and building control coordination, with internal finishes and specialist site work itemised where applicable.
The main cost factors are overall size, bespoke glass shapes, thermal and solar-control specification, roof and door spans, structural steel, foundations, site access, planning requirements and internal finish.
Many Glass Extensions fall under Permitted Development Rights, but restrictions apply in conservation areas and for listed buildings. Room Outside provides full planning guidance with every project.
Yes. Our Glass Box Extensions feature New Generation Glass technology with advanced thermal breaks and low-e coatings, with the potential to achieve A++ energy ratings.
A typical Glass Box Extension takes 12 to 16 weeks from initial consultation to completion, including design, structural engineering, foundations, installation and finishing.
Depending on product and location, we offer up to a 10-year structural guarantee backed by independent UK insurers, alongside separate product-specific warranties.
Get Your Free Glass Extension Quote
Book a free home design consultation with our specialist team. We will discuss your vision, assess structural requirements, and provide a detailed quotation.
Prefer to talk? Call 01243 538999.