Glazed Extensions: The Complete UK Guide

Glazed extensions cover everything from a bolt-on Victorian-style conservatory to a fully insulated garden room with its own kitchenette. In 2026, UK prices across the family run from around £8,000 for a modest lean-to conservatory to £60,000 or more for a large, high-specification garden room or orangery, with roof lanterns and rooflights sitting at the cheaper end as an upgrade to an existing extension rather than a standalone structure.

The right choice depends on how the space will be used, whether it needs to be heated and lived in year-round, and how much planning risk you’re willing to take on. But the cost gap between the options is not mostly about glass and frames. It is about which regulatory regime the finished room lands in. A conservatory that keeps its separating doors is exempt from the building regulations altogether. Take those doors out and the same structure becomes an extension, has to hit extension-grade U-values, and can cost several times as much. Everything below is organised around that single fork in the road.

The Five Options, and What Separates Them

There is no legal definition of an orangery or a sunroom. Manufacturers use the words loosely, and two firms will happily sell you the same building under different names at different prices. What the law recognises is much narrower, and it is worth learning the three distinctions that actually change what you pay.

The first is attached or detached. An attached glazed room (conservatory, orangery, sunroom, porch) is an extension to the house and is assessed under Class A of the permitted development order. A detached one (garden room, garden office, summerhouse) is an outbuilding under Class E, with different and generally tighter height limits but a more generous approach to floor area.¹

The second is glazed or solid roof. A conservatory in the regulatory sense is a substantially glazed structure. Put a solid insulated roof on it and it stops being a conservatory: it becomes an extension, with a roof that has to perform and a structure that has to be shown to carry the extra load. This one change is a common cause of a project going over budget.

The third is thermally separated or open to the house. This is the distinction that decides whether the energy efficiency requirements apply at all, and it is the one homeowners give away most casually, usually at the point where a salesperson suggests losing the doors to “open it up”.

Everything else, style, frame material, roof pitch, whether the ridge has a finial on it, is a matter of taste and price. Our comparison of an orangery against a conservatory and a full extension sets out the aesthetic and resale differences, but the three questions above are the ones that move the number on the quote.

The Conservatory Exemption, and Exactly When You Lose It

The exemption is real and it is generous, but it is conditional on five things at once, and it is not written where most people look for it.

Schedule 2 to the Building Regulations 2010 exempts, at class 7, the erection at ground level of an extension consisting of a conservatory, porch, covered yard or covered way, or a carport open on at least two sides, where the floor area does not exceed 30 square metres.² That is the whole of the structural exemption. Regulation 9 is what gives it legal effect, and it also carves two things back in that still apply even to an exempt building: the water efficiency requirements where a water supply is shared with the dwelling, and Part P electrical safety wherever the building takes its electricity from the house.³ A garden office or conservatory wired off the house consumer unit is therefore notifiable electrical work whatever the shell’s status.

Separately, Approved Document L sets out when a conservatory or porch is exempt from the energy efficiency requirements specifically. All five of the following must apply: the extension is at ground level; its floor area does not exceed 30 square metres; the glazing complies with Part K; any wall, door or window separating the extension from the dwelling has been retained, or if removed has been replaced with a wall, door or window; and the dwelling’s heating system is not extended into the conservatory or porch.⁴

The Planning Portal restates the same test in the form building control inspectors actually apply on site: single storey, under 30 square metres, separated from the house by external quality walls, doors or windows, with an independent heating system under separate temperature and on/off controls, and compliant glazing and fixed electrics.⁵ It also flags the trap that catches most projects, which is that any new structural opening between the conservatory and the existing house needs building regulations approval in its own right, even where the conservatory itself is an exempt structure.⁵

So the exemption is lost in four practical ways, in rough order of how often it happens:

1. You remove the separating doors. Approved Document L is explicit that if the thermal separation is removed, or the dwelling’s heating system is extended into the space, the conservatory should be treated as an extension.⁴ At that moment the whole of the extension guidance applies, including the U-values and the glazing area cap below.
2. You run a radiator off the house system into it. Same outcome. An independent electric heater with its own controls keeps the exemption; a spur off the central heating does not.
3. You go over 30 square metres. The class 7 exemption simply stops. Note the nuance here: a conservatory that is too large but still properly separated is treated as a non-exempt conservatory rather than as an extension, so its elements must meet the Approved Document L limiting U-values but the 25% glazing area cap does not apply to it.⁴ That is a meaningfully cheaper outcome than removing the doors.
4. You put in a solid roof. The structure is no longer substantially glazed, so it is no longer a conservatory. It is an extension, and the existing base and frame have to be shown capable of carrying the new roof load.

That last one is why conservatory refurbishment costs vary so widely: replacing a polycarbonate roof with an insulated one on a twenty-year-old frame is a structural job, not a cladding job.

Permitted Development: The Limits That Actually Bite

A conservatory is treated as an ordinary home extension for planning purposes and falls under the same Class A rights as any other single-storey addition, with the limits applying to the total enlargement including anything built before.⁶ The rules apply to houses only, not to flats or maisonettes, and not where an Article 4 Direction or a planning condition has removed the rights.⁶

The numbers in Class A itself, rather than in a council’s summary of it, are these. Buildings within the curtilage, including the extension and any outbuildings, must not cover more than 50% of the curtilage of the original house. A single-storey rear extension may not extend more than 4 metres beyond the rear wall of a detached house, or 3 metres for any other house, and may not exceed 4 metres in height. Eaves within 2 metres of a boundary may not exceed 3 metres. A side extension may not exceed 4 metres in height, may not have more than one storey, and may not be wider than half the width of the original house. On article 2(3) land, which includes conservation areas, side extensions and multi-storey rear extensions are removed from the class entirely.⁷

The doubled depths, 8 metres for a detached house and 6 metres for any other house, are available only through the larger home extension procedure, which requires you to notify the local authority so it can consult the adjoining neighbours before you build.⁸ That is prior approval, not a free right, and it is worth reading our guide to permitted development alongside the rear extension rules before assuming the bigger figure applies to you.

For a detached garden room the governing text is Class E, which permits any building or enclosure within the curtilage required for a purpose incidental to the enjoyment of the dwellinghouse. The limits are: single storey only; maximum height 4 metres with a dual-pitched roof, 3 metres otherwise, and 2.5 metres if any part is within 2 metres of a curtilage boundary; eaves no higher than 2.5 metres; nothing forward of the principal elevation; the same 50% curtilage cap; no verandah, balcony or raised platform; and nothing within the curtilage of a listed building.¹ “Incidental” is the word that does the heavy lifting: a garden room used as a separate dwelling is not incidental and falls outside the class altogether, which is the point our guide to garden room planning permission works through in detail.

Building regulations for detached outbuildings follow a different threshold again. Approval is not normally needed where the floor area is under 15 square metres and there is no sleeping accommodation. Between 15 and 30 square metres it is still not normally needed, provided there is no sleeping accommodation and the building is either at least 1 metre from any boundary or constructed substantially of non-combustible materials.⁹ Sleeping accommodation removes the exemption at any size, which is why “annexe” and “garden room” are not interchangeable words.

If any of this is close to the line, a lawful development certificate is the cheap way to fix your position before you build, and if you are in a conservation area you should assume rights have been trimmed. Article 4 of the order lets the local planning authority direct that permitted development rights do not apply to a specified area or a particular development, which is exactly how conservation areas are commonly managed.¹⁰ Our guide to how big you can build without permission covers the arithmetic in more detail.

Part L: The Numbers Your Quote Has To Meet

Once a glazed room is an extension rather than an exempt conservatory, Approved Document L Table 4.2 applies to every new element in it. The limiting U-values, in W/(m²·K), are 0.15 for a roof, 0.18 for a wall, 0.18 for a floor, 1.4 for a window (or Window Energy Rating Band B), 2.2 for a rooflight, and 1.4 for doors, with more than 60% glazed doors allowed on Doorset Energy Rating Band C.⁴ The timber window concession of 1.6 ended on 14 June 2023, so a quote still leaning on it is out of date.⁴

There is also a glazing area cap that catches people out, because it is the opposite of what a glazed extension is for. The total area of windows, roof windows, rooflights and doors in an extension should not exceed 25% of the floor area of the extension, plus the area of any windows and doors that no longer exist or are no longer exposed because of the extension.⁴ The second half of that sum is what makes a fully glazed rear extension possible: knocking out a pair of French doors and a window buys you their area back. If you cannot make the sum work, Approved Document L offers two alternatives: an area-weighted U-value calculation showing the extension performs no worse than a compliant one of the same size and shape, or a full Standard Assessment Procedure calculation against a notional extension.⁴ Both cost money in fees, which is one reason a glass-heavy design carries a design cost as well as a build cost. Our guide to glass box and bifold extensions covers what that looks like in practice, and extension building regulations covers the wider approval process.

Two smaller Part L points are worth knowing. If you enlarge an existing window or create a new one in the house itself, the total area of openings should not exceed 25% of the dwelling’s total floor area, or compensating measures should be taken.⁴ And replacing glazing, or a window or door within its existing frame, is not “providing a controlled fitting” at all, so it does not have to meet the energy efficiency requirements.⁴

Heat, Glare and Overheating

Approved Document O deals with overheating, and the first thing to say about it is that it does not apply here. Its guidance applies to new residential buildings only.¹¹ An extension or conservatory on an existing house is outside its scope, and any installer who tells you a conservatory must comply with Part O is wrong.

It is still the best published benchmark for how much glass a room can take before it becomes unusable in July, because it is the only one derived from modelling rather than opinion. For a cross-ventilated room in a moderate risk location, which is all of England outside the listed urban and suburban parts of London, Approved Document O caps glazing at 15% of floor area for a south-facing largest glazed façade, 18% for north or east, and 11% for west, with the most glazed room allowed 30% south, 37% north or east, and 22% west.¹¹ Take the cross-ventilation away and the whole-building caps for a moderate risk location stay the same, but the allowance for the most glazed room collapses: 15% south, 26% north or east, 11% west.¹¹ In other words, most of the tolerance for a heavily glazed room comes from being able to open something on the opposite side of it. In the high risk London locations, glazed areas between north-east and north-west via the south should additionally be shaded, by external shutters with ventilation, glazing with a maximum g-value of 0.4 and minimum light transmittance of 0.7, or a 50 degree overhang cut-off on due south façades.¹¹

A typical conservatory is glazed at several multiples of those percentages, which is precisely why they overheat. The practical responses, in order of effectiveness, are external shading, then solar-control glass, then internal blinds, then ventilation. Conservatory blinds and their cost compares pleated, pinoleum and perfect-fit options, and blinds against curtains and shutters is the wider comparison. Cross-ventilation, meaning openings on opposite faces, is free and is the single design decision that does most for summer comfort.

The Glass Itself: Safety, Thickness and Thermal Spec

Glazing in a critical location must break safely, be robust, be in small panes, or be permanently screened. The critical locations are the shaded zones in Approved Document K Diagram 5.1: in a door, any glazing below 1500 mm from floor level; in a side panel within 300 mm of the door edge, glazing below 1500 mm; and elsewhere in walls and partitions, glazing below 800 mm.¹² Safe breakage means Class 3 of BS EN 12600:2002 or Class C of BS 6206, and Approved Document K refers to the higher Class 2 or Class B for a pane wider than 900 mm in a door or door side panel.¹² In small panes, annealed glass should be a minimum 6 mm nominal thickness, panes should be a maximum 250 mm wide and no more than 0.5 m² in area.¹²

For the robustness route, Approved Document K gives maximum dimensions for annealed glass in large four-edge-supported areas: 8 mm up to 1.10 m by 1.10 m, 10 mm up to 2.25 m by 2.25 m, 12 mm up to 3.0 m by 4.50 m, and 15 mm for any size.¹² Those are the numbers behind the jump in price when a glazed gable or a structural glass panel goes past about 2.25 metres in either direction. In practice almost every domestic glazed extension uses toughened or laminated units rather than annealed glass, but the thickness bands still explain the cost curve.

On thermal specification, the Glass and Glazing Federation’s own comparison is the most useful public data. A 28 mm double glazed unit (4-20-4) with one low-emissivity coating and a warm edge spacer gives a centre pane U-value of 1.5 W/m²K air filled or 1.2 argon filled, and a whole window U-value of 1.6 or 1.3 respectively.¹³ A 28 mm triple unit with only one low-E coating is actually worse than the argon-filled double, at a whole window 1.4; you need two low-E coatings before triple glazing pulls ahead, and a wider unit before it pulls ahead decisively.¹³ The GGF states plainly that there is no benefit in triple glazing, subject to cavity width, unless two panes of low-E glass are used.¹³ Our note on glazing U-values, low-E and spacers explains the terms, and double against triple glazing covers whether the upgrade pays.

One certification point that surprises people: porches and conservatories cannot be registered on a glazing competent person scheme at all. Registrable work is the replacement of a complete window or door unit within the thermal envelope of an existing dwelling. New builds, conservatories, porches, enlarged openings and installations outside the thermal envelope are excluded, and so are repairs such as swapping a misted sealed unit.¹⁴ So the absence of a FENSA or Certass certificate on a new conservatory is correct, not a warning sign. Work outside the scheme goes to building control instead, and that is where your completion certificate comes from.

Conservatories: Cost and Types

A conservatory is defined by its largely glazed roof and walls, and is usually built under permitted development provided it stays single-storey and within footprint limits.⁶ Our full breakdown of conservatory prices by style, lean-to, Victorian, Edwardian and P-shape, covers what drives the £8,000–£35,000 range and the glazing specification worth insisting on.

Older conservatories rarely perform well against modern Part L standards; if yours has a failing polycarbonate roof, conservatory refurbishment costs covers swapping it for a solid insulated roof versus a full refit. Whatever the roof, solar gain is the main comfort problem in summer; conservatory blinds and their cost compares pleated, pinoleum and perfect-fit options for heat and glare control.

The cheapest way to make a conservatory usable in winter is not more heating. It is to keep the thermal separation, heat the space independently and intermittently, and accept it as a three-season room. Under-slab insulation and underfloor heating turn it into a year-round room, but at that point you are building an extension and should price it as one.

Orangeries and Sunrooms

An orangery sits between a conservatory and a full extension: more brick, a flat roof with a glazed lantern, and a stronger case for year-round use at £30,000–£60,000. Our comparison of an orangery against a conservatory and a full extension sets out the cost, thermal performance and resale differences.

Because an orangery’s roof is mostly solid, it will rarely qualify as a conservatory for the exemption, so assume the Table 4.2 U-values apply and that the glazed area sits inside the 25% calculation. That is not a reason to avoid one. It is a reason to make sure the quote you are comparing has priced an insulated roof, a compliant floor build-up and thermally broken frames rather than assuming an exemption it will not get.

A sunroom takes the opposite approach, mostly glazed walls in a timber or aluminium frame, prioritising garden views over thermal mass. Our sunroom cost guide explains the gap between a prefabricated kit and a fully installed, Part-L-compliant structure. If the glazed wall is the whole point, compare bifold doors against sliding patio doors before committing: sliders generally give a better U-value and more glass per pound, bifolds give a wider clear opening.

Garden Rooms: A Detached Glazed Space

A garden room is a standalone glazed building rather than an attached extension, with different planning and value trade-offs. Our garden room cost and planning guide covers the full range from a basic insulated box to a high-specification year-round space, and garden room planning permission rules explains the Class E limits that keep most builds consent-free.

The height limits are what usually force a redesign. Within 2 metres of a boundary the whole building is capped at 2.5 metres, which after a floor build-up and a roof leaves an internal height that feels tight.¹ Keeping every part of the building more than 2 metres from the boundary buys back 500 mm of height with a flat roof, or 1.5 metres with a dual-pitched one.¹ That single decision changes the feel of the room more than any amount of glazing does.

Weighing a detached garden room against extending the house itself? Garden room vs extension compares cost, added value and disruption. And if the room is destined to be a home office, garden office vs garden room covers the extra insulation, power and broadband specification that working from it daily demands. Remember that even where the shell is exempt, the electrical supply from the house is notifiable under Part P.³

Roof Lanterns and Rooflights

Not every glazed addition needs to be a whole new room, a roof lantern or rooflight can bring the same light into an existing flat-roof extension or loft. Our roof lantern cost guide covers sizes, frame materials and the £1,500–£8,000+ fitted range.

Fitting one almost always means cutting away part of one or more rafters or joists, so new support has to be introduced, usually two timbers fixed together and spanning the new opening on each side.¹⁵ The unit itself must meet the Table 4.2 rooflight U-value of 2.2, and the room it serves still needs both purge and background ventilation, which the rooflight can provide; ventilation of the roof void has to be considered separately.¹⁵ Those three requirements, structure, U-value and ventilation, are the difference between a lantern that passes building control and one that does not.

For the wider family of options, rooflight materials and types compares flat rooflights, pitched roof windows and lanterns on U-value and cost, and skylights vs roof lanterns vs roof windows is the quickest route to the right term for your roof shape. If the glazing is going into a loft rather than a ground-floor extension, loft conversion windows and skylights covers the fire-egress rules specific to that context.

Choosing Between Them

The decision, stripped down, is a trade between three things: how much of the year you want to use the room, how much regulatory cost you are willing to absorb, and how much of the garden you are willing to give up. A conservatory buys you the most floor area per pound, but as a thermally separated room it is realistically a three-season space unless you spend again on it. An orangery buys year-round use, and pays for it by landing squarely inside the extension rules and their U-values. A garden room buys a quiet, separate room without touching the house, at the price of walking outside to reach it and of giving up garden.

Ask About Part L Before You Compare Quotes

A conservatory quote that is thousands cheaper than a rival’s may simply leave out the insulated roof, upgraded glazing or separating doors that decide whether the room stays exempt or has to meet Part L. Ask each contractor to confirm in writing whether their price assumes the conservatory exemption or full Part L compliance before comparing line by line.

The Expensive Mistakes, and Why They Happen

Removing the separating doors after completion. One of the most costly mistakes in this category, and an easy one to make, because it happens after the builder has gone. Opening the conservatory to the house removes the thermal separation, converts the room into an extension in the eyes of Approved Document L, and makes the whole structure retrospectively non-compliant.⁴ A buyer’s solicitor may well ask about it, and the fix is usually to reinstate doors or to rebuild the roof and walls to extension standard.

Assuming an exemption survives a solid roof. A replacement solid roof changes both the regulatory status and the loading. The existing frame and foundations were designed for a light glazed roof and were probably never inspected, because the original build was exempt. Get the structure assessed before ordering the roof, not after.

Buying on price without checking the glazing area sum. Two quotes for the same footprint can differ by thousands purely because one has priced the 25% glazing calculation and the compensating measures or SAP assessment needed to pass it, and the other has assumed it will not be checked.⁴

Building right up to the boundary. For a detached garden room, being within 2 metres of a boundary cuts the height limit to 2.5 metres and, once you pass 15 square metres, forces either a 1 metre setback or substantially non-combustible construction to keep the building regulations exemption.⁹¹ Both are cheaper to design around than to fix.

Treating the summer as an afterthought. A south or west-facing glazed room with internal blinds alone will be unusable on hot days. Shading, glass specification and cross-ventilation are design decisions, not accessories, and they cost far less at the drawing stage.

Where to Start

If budget is the main constraint, start with the conservatory cost guide, still the cheapest way to add a glazed room. If you want a standalone, year-round space, the garden room cost and planning guide is the better starting point. And if you already have an extension but want more light in it, the roof lantern cost guide is the quickest, cheapest upgrade of the three. Whichever route you take, read building regulations for homeowners first, so you know which regime your room will end up in before you sign anything. If a glazed room turns out to be the wrong answer and you need a solid-built addition instead, the house extension guide covers every other kind of extension.

A porch is the smallest member of the glazed family and sits under the same class 7 exemption as a conservatory: adding a porch walks through types, building regulations and quotes, and porches and canopies compares an enclosed porch with an open canopy. If the room has to be lived in all year, price it against a solid build. The single storey extension cost guide sets out what a conventional ground-floor addition costs, single storey extension ideas and house extension design ideas show how rooflights and glazed walls bring the same light into one, and a kitchen extension is where most glazed rear walls end up. On a terrace or semi, a side return extension or a wrap-around extension often gains more usable floor than a conservatory on the same plot. For a heavily glazed frame with a solid structure behind it, compare an oak frame extension with a SIPs extension. And if what you need is a bedroom or storage rather than a sunny room, a bedroom extension, a double storey extension or a garage extension is the better route.

The rules in this note are those that apply in England. Scotland, Wales and Northern Ireland have their own building standards and permitted development rights.