Perfect extensions

Cathy Strongman says Londoners planning to extend their homes should follow a simple golden rule: go for quality, not grandeur
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Among the most exciting architectural projects in London are many that are hidden from public view. Throughout the capital, home-owners are adding precious extra space by converting their side returns into dramatic glass extensions and conservatory rooms which are showcasing the talents of architects and equipping our homes for 21st century living.

Most ground-floor extensions consist of open-plan kitchen and dining areas connected to the garden through generous amounts of glass. Many properties in London are ideally suited to this kind of expansion. The Victorians left a legacy of fantastic innovations but they also built row upon row of L-shaped houses. The kitchen usually extends out into the garden, leaving a dead space that more often than not has become a dingy patio or a dumping ground for family clobber.

'We are not talking shoddy PVC extensions where wicker furniture goes to die'

Building a side extension to incorporate this valuable space is an obvious solution. You get more internal space without eating into the best bit of the garden, and with the injection of a glazed roof or generous roof lights you can bring natural light into the home.

For those with more grandiose plans, properties can be extended across the back of the house and into the garden. We’re not talking shoddy PVC extensions where wicker furniture goes to die. Bold contemporary additions can be built in a variety of styles. You could even create a double-storey extension with a new bedroom above. By incorporating glazed walls or roofs it is possible to enjoy large amounts of light and many people opt for generous glass doors that either fold or slide back to blur the boundary between indoor and outdoor living.

Your piece on the side

The cost of extending your home varies enormously, of course, but generally depends on the floor area and the design specification. Side extensions are the cheapest, starting at about £20,000. But a cutting-edge design — perhaps a glass box kitted out with a top-of-the-range kitchen and connecting to a re-landscaped garden — could set you back more than £130,000. Still, with moving costs often exceeding the £100,000 mark, an extension can make financial sense.

According to Halifax bank, a substantial extension can add more to a home’s value than any other home improvement. And, says Tom Tangney, a partner in estate agent Knight Frank: “If the project is well done, you could double your money on a ground-floor extension.”

With £130,000-plus to spend, an extension can provide an exciting architectural project
With £130,000-plus to spend, an extension can provide an exciting architectural project
Ground-floor extensions are generally easy to get planning permission for — many will fall within “permitted development rights” and more planning-law changes coming in October are likely to free up the permission process even further. These extensions are less disruptive than a loft or basement extension, too.

The length of the build depends on the extent of the project and the design but the back of the property can be boarded up, leaving the rest of the house habitable. Building an extension offers the chance to get stuck into an architecturally exciting project without the expense and work required when building an entire house.

If planned, designed and built correctly you can gain space and add value to your home with an extension, while making your own contribution to London’s architectural transformation.

First do your sums...

A simple conservatory can be added for as little as £10,000 to £15,000, according to The Federation of Master Builders, but if you really want to add value to your property it is worth employing an architect and investing in a more permanent side or back extension. Prices for a modest side extension to a Victorian property start at about £20,000 but could easily drift up to £75,000. A single-storey back extension can cost anything from £35,000 to £100,000 or more. If you build a first-floor extension over an existing part of the house, expect to pay even more as the foundations may well need to be re-enforced.

The cost of your extension should relate to the value of your property. If your home is worth £1 million then it is worth spending upwards of £50,000, but for more modest properties the investment should be proportionately less. Investigate your target buyers and improve the house to suite their requirements without pricing it out of the market. Most areas have a ceiling price — buyers won’t go above this no matter what you do to your house — so check with local estate agents before you decide on the scale and expense of your project.

Also ensure that the new extension is well integrated with the rest of your property. Tie in floor levels and surfaces and ensure the new space is heated.

A shoddy extension that looks like a temporary addition will put buyers off, rather than bump up your asking price. “Open-plan kitchen/diners are really popular with buyers,” says Knight Frank’s Tom Tangney. “I’ve seen lots of people create these spaces by extending on to the side or the rear. Floor area is in high demand in the capital and with a successful side or back extension, when you come to sell, you can get back double the amount the extension cost you.”

Follow the rules and regulations

Many back and side extensions can be built under permitted development rights, which allow adding volume to your home up to a certain limit. The cut-off point is 1 January, 1948, when the Town & Country Planning Act came into effect. Anything built before that date is taken to be part of the existing house. Permitted development rights enable you to extend by up to 15 per cent of the existing house in volume, or 70 cubic metres, whichever is the greater. For terrace houses, this is reduced to 10 per cent, or 50 cubic metres. You will face other restrictions in conservation areas. Listed buildings need planning permission whatever you want to do.

Building regulations approval is needed for all extensions. New regulations concerning heat loss are particularly relevant for those extensions that use a lot of glass. Your architect will be able to advise you on planning permission and building regulations and more information can be found on the Communities and Local Government website. If there is any doubt, consult the planning department of your local council. You should also be aware that the rules governing what householders can do without the need to apply for planning permission are to change on 1 October 2008. Any changes are likely to make building an extension easier than it is already.

A stunning glass basement extension
A stunning glass basement extension

Use glass that passes examination

Glass is the defining feature in many modern extensions, with large sliding doors, extensive roof lights and even entire structures made from the material. Incorporating glass into the roof and walls of your extension is a brilliant way of introducing more light into your property and connecting your living space with the garden, but it is important to select high-performance glass in order to meet building regulations and to create a comfortable living space.

Specify low-emission (Low-E) double- or triple-glazed panels, which are coated with a thin layer of metal oxide to retain internal heat. Panels filled with an inert gas, such as argon, will conduct less heat to the exterior than panels filled with air.

To prevent heat gain in the summer you can install semi-reflective glass or apply a reflective film to your panes. Alternatively, design your extension with an overhanging roof, which will allow lower-lying winter sun to pass through the glass but will block out the harsher rays of a summer sun. The clever planting of trees that shed their leaves in the winter but create shade in the summer will create a similar effect.

Grow grass on top

Eco measures are now part of building regulations. Known as Part L regulations, they set out stringent rules for greener homes, though you can go further by installing additional insulation.

“A ‘green’ roof or a vertical garden system on one of the walls of your extension is a fantastic way to improve thermal and acoustic insulation while bringing more beautiful, green things into your life,” says Berit Lake, design manager at Eco Age ( “They will also prevent water run-off and encourage biodiversity.”

If you opt for a timber-clad extension make sure the wood is from a certified sustainable source. Coppiced sweet chestnut is a good example, or certified sustainable Western Red Cedar. Timbers such as these, which are high in oil content, won’t need treating and weather to a subtle silvery grey.

Because you will already have scaffolding in place, you could also consider installing solar hot water or photovoltaic panels on the roof. Lake advises checking out the latest transparent solar panels, which allow light in while harnessing energy from the sun.

Oliver Heath of Blustin Heath Design recommends installing underfloor heating in a new extension, as this is more energy-efficient than standard radiators.

Finally, make sure you have a responsible waste-management plan in place. Some construction waste, such as structural timber, could be reused within the new extension whereas old windows, floorboards and even the back boxes for lights you have removed can be sent to a reclamation yard.

Generous amounts of glass and good-quality materials will bring more natural light into your home
Generous amounts of glass and good-quality materials (here, oak is used for the frames, cedar for cladding and poured-concrete for the floor) will bring more natural light into your home and impress future buyers

Floors and windows

An essential component of your new extension will be its floor. Oliver Heath, of Blustin Heath Design, says: “Choose something hard-wearing and easy to clean and that works well with underfloor heating, such as floor tiles or stone.”

Crucially, pick a surface that will match well with the rest of the house to connect old and new seamlessly, he adds. “Also, position the kitchen at the back of the room and the dining area closest to the garden, so you can socialise with guests while cooking and diners can enjoy the garden view.”

If your design incorporates a lot of glass, your room will become a lightbox at night, which can cause privacy problems.


* RIBA: (for a list)
* Phil Coffey Architects: 020 7549 2141;
* Paul Archer Design: 020 7729 2729;
* Plastik Architects: (020 7713 0728;

* Glass and Glazing Federation (GGF): 0870 042 4255;
* GlasSpace: 01268 782307;
* Abode Window Films: 0116 278

Planning and building regulations:
* Department of Communities and Local Government: 020 7944 4400;

Eco upgrades:
* Eco Age: 020 8995 7611;

Interior design:
* Blustin Heath Design: 020 7739 6416;
* Velux blinds: 0800 581425;
* John Lewis: 0845 604 9049;

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