As a building style, the average conservatory normally enjoys all the mock-Victorian characteristics it can muster: finials, mouldings, crestings. Bung an aspidistra into a corner, ramp up the raffia, and you'll feel quite the fusty gentle-person.
But this kind of genteel, nostalgic garden room, so popular in the 1980s, is giving over to a bracing new generation of glass extension. Indeed, if the earlier era of glazed extensions referenced the high Victoriana of Joseph Paxton and the Crystal Palace, the new minimal extension is a take on the mid-century modernist Mies van der Rohe's Farnsworth House of 1951 in Illinois – the stylistic prototype for the shiny cubes that are now proliferating from the backs of houses in British cities. Side returns, basement wells and patio spaces across the land are being wrapped in glass, bringing a transparent high-tech hit to the sooty brick backs of British houses.
Architect Paul Archer, of Paul Archer Design, has become the brand leader in glass extensions and he cites three main reasons why the glass box has become the pre-eminent statement of the age. Firstly, it charts a changing taste in domestic design: we no longer demand a kind of domestic greenhouse and instead look more for domestic-scale modernity. Archer's glass cube in Wallace Road, north London, is a case in point, jutting elegantly from the London brick.
Secondly, the current crop of glass cubists have been driven by developments in technology and engineering which enable glass to be used as a wall as well as a window. "From a structural point of view, the breakthrough came around 1990 when glass became tougher and architects started using it as a structure in its own right," Archer says. This means that glass structures no longer need the framing that they once did, with big wooden bars or, at worst, ugly white uPVC.
And glass now has a much better thermal performance and no longer represents an energy drain. Archer says that within the last decade, glass structures have gained a similar U value (the measure of how well a structure keeps the heat in) to a recently built cavity wall. "For example, we're doing a zero carbon house and about 40 per cent is glazed," Archer says.
Indeed, one of the many woes of our current crop of volume new-builds is that in response to building regulations, windows have shrunk. "Not necessary," Archer says. "With good design there wouldn't be a need for smaller windows." But glass does require a bit of specialist knowledge and, as Archer says, "it's quite a complicated material, with the potential for unpredictable behaviour down the line". Most notably, this comes as a result of something called "nickel inclusion".
"Glass has hidden faults with fine particles of nickel," Archer says. "These explode unless you build in redundancy or back-up [this happened at Waterloo Station in 1999]." To counteract this, a glass beam will often use three pieces of glass laminated together to afford an error margin.
Anyone considering a glass cube should keep in mind a few other considerations. Archer says a glass roof should be strong enough "to carry the load of a burglar". And there's the minor safety aspect of collision, which is why glass walls are usually marked in public buildings. "I haven't noticed it being a problem on a domestic scale but I have seen a couple of dogs walk into glass walls," Archer says.
More importantly, there's privacy to consider: that of your neighbours – and your own. "A glass extension gives traditional Victorian London houses what they crave most, which is light," Lindsay Cuthill, of Savills agents in Fulham, says. "But owners should consider that in a built-up area, good visibility outwards is also good visibility inwards. You can end up feeling like you are living in a goldfish bowl."
Archer says this can be countered by a judicious use of blinds and by investigating specialist glass and films: "There's a product called Lumisty, which has a polarising film, for instance. You can control the angle you see through it: look at an angle, and it's opaque; look straight and it's transparent."
The visibility factor is often part of the planning equation and while extensions used to be permitted development, Archer says that anyone wanting to build their own glass cube should get planning permission.
Fortunately, this is now easier. "There's a planning perception in favour of visually lightweight structures," he says. Incidentally, in terms of listed buildings and conservation zones, an avowedly modern add-on is preferred by some planners and conservation officers, as it renders the older building more legible with the boundary between old and new easy to discern. Nick Churton, of London agents Mayfair Office, says: "Planners are now largely sympathetic to this idea, and traditional buildings can be given new life. It's great, as this would have been far from easy in the past. Glass lends itself to extending period houses: in fact, I think it's the ideal material."
Still, the most worrisome aspect is the heating, that a glass cube will be too cold in winter and too hot in summer. "You have to look at the orientation and think design," Archer says. In his Wallace Road extension, for example, he added small ventilation ducts at the back that relieve the hot air of the summer months. It's a case of thinking about which way your glass cube faces and how it relates to the rest of the house. "A great added advantage of glass extensions is that they can illuminate adjoining rooms that may have been on the dark side before being opened out," Ed Cunningham, of the agents Knight Frank, says.
There are a few other concerns. "Be careful not to overextend and drastically reduce the garden," Hugh Greenhouse, a RICS surveyor and managing director of Homebuyer Online, says. "Make sure that the extension works for the property and assess whether the extra rooms are going to be of decent proportions."
Greenhouse has seen a glass-roofed kitchen 27ft long but 4ft wide. Some might consider going the whole dramatic route and making a glass curtain wall across two storeys with a mezzanine floor – a way to bring a dramatic, atrium style to your home. "Only a few people go for it, as you tend to lose some floor space," Archer says.
So how much are they? Not as much as people may think. The glass itself can cost a fair amount, but as it's fabricated off-site, there isn't usually a great labour cost to putting it in. "The 'from' price is probably about £100,000," says Archer, who estimates that most jobs will cost £250,000 to £300,000. Then, the question is whether that will transfer to your property's value. Archer says it will, "provided it's good design". He adds that Wallace Road has just sold for 20 per cent more than a similar near neighbour. "I like to think it was down to the glass extension."
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