Look down on your neighbours: How city gardeners can broaden their horizons with a roof terrace

As summer approaches, it's time to start thinking about outside space. We are not all lucky enough to have acres of garden, but with a little planning even the smallest area can reap horticultural and financial rewards.

Rooftops and terraces can provide accessible outdoor retreats high above neighbours' prying eyes. They will almost certainly be suntraps and can provide great views. They can even add 10 per cent to the value of a home, according to a recent study by Bradford and Bingley. But it takes some forward-planning: roof gardens take longer to design and build than their ground-level equivalents and can go horribly wrong if you simply try to recreate what you would do in your backyard.

Will it work?

Check planning guidelines in your area, particularly if you need to increase the height of walls. "Planning regulations vary from council to council and neighbours are often very protective of their privacy," says Arabella Gurney, creative director at home design company Blaze & Co (www.blazeandco.co.uk). "However, if you've been using it as a roof terrace for more than seven years, you should be able to apply for retrospective permission."

A roof garden needn't cost much, insists Sue Amos, a garden designer at Moss Space (www.mossspace.co.uk). "A slick decked terrace with mature trees and shrubs, bespoke planters and bubbling features could start at £30,000, but a collection of recycled pots, decking squares, plants from seed swaps and boot fairs can be an affordable option, with the added benefit that you will have done it all yourself on the cheap."

If it is a big job, two major cost implications are often missed. First, allow a big chunk of your budget for railings: most people like glass, but this costs at least £200 per metre, says Ruth Marshall, director of Cool Gardens Landscaping (www.coolgardens. co.uk). Second, it's expensive running water pipes to the roof, but you should consider installing a tap to avoid walking up and down stairs with watering cans.

Once you've identified your ideas and budget, you may want to consider getting a structural engineer to check the load-bearing capacity of your roof and also make sure it is watertight. "Wet soil is heavy, and once the planters and plants are in place, you won't be able to move things to find a leak," says Amos.

If the job feels overwhelming, call in a garden designer, who can answer all your questions, as well as make any planning applications on your behalf. But make sure they have experience of designing and building roof gardens.

Flooring

Typically, you will need a floating construction with as little weight resting on the waterproof membrane as possible – and ensure you don't breach the damp seals, says Marshall. It's not out of the question to lay grass on top, but its soil base is weighty, so get advice beforehand.

Adolfo Harrison, garden designer at Templeman Harrison (www.templemanharrison.com), suggests using artificial turf instead. "It used to be really naff but is improving all the time. Some even have fake dead thatch and look incredibly realistic."

Amos agrees: "Fake grass can be particularly luxurious and comfortable – great for those who just want to loll around or play. Decking is another option: it's smart, long-lasting and gives a beachside feel, as well as being lovely underfoot. A good hardwood will weather to a beautiful soft silver."

Colour is more important than you might think, she adds: "Since roof gardens get full sun and high UV, avoid dark colours such as slate as they can get too hot to walk or sit on. Alternatively, light-coloured paving can create a lot of glare."

Sarah Bevin, a designer for Urban Roof Gardens (www.urbanroofgardens. com), prefers paving to decking. "It's more resilient and easier to maintain, though make sure you have adequate support for the tiles in place and that they have a slip resistance."

You can always mix and match, says Chris Maton, a garden designer at Olivebay (www.olivebay.co.uk). "In a roof terrace and courtyard roof garden I recently designed, I used a combination of hardwood decking and natural stone paving to define different areas. Using different materials adds interest to a design."

Plants

Roof gardens can be a far more extreme environment than most people realise, says John Wyer, landscape architect and garden designer at Bowles & Wyer (www.bowleswyer.co.uk), "so if it's a windy day, you'll get a howling gale on the roof and if it's sunny, it can get really, really hot. People often grow things like roses on a roof, but roses don't like this kind of exposure. Nor do they like being in containers."

His advice when choosing plants is to think Mediterranean: "Contrary to what people think, they will take high levels of exposure and cold. Number one on my list for roof gardens is olive trees. We recently planted one on the 13th floor of a London block. Fan palms are good, too. They suffer some damage in the winter, but they always bounce back and survive in the long term."

Seaside plants are worth considering as well, adds Wyer. "Tamarix and griselinia work well and for shrubs, think about Mediterranean hillsides, where they have sunshine, exposure and shallow soils, just like a roof garden. Rosemary and lavender are good examples. In more sheltered positions, where it's more shady, box and yew grow well, including in containers: perfect for that clipped, formal look. If you want to bring in bursts of colour or texture, do it year on year with summer bedding plants."

Amos's preference is for wild and exotic plants. "Anything from New Zealand, Chile or South Africa works a treat – phormium, libertia, the shorter Pseudopanax crassifolius, the dwarf Pinus mugo or even the native Scots pine," she says. Because roof gardens are often small and the walls are a key part of the space, consider climbers including Muehlenbeckia complexa and honeysuckle, as well as star jasmine for wafts of evening perfume.

Don't hold back on plants, says Bevin. They don't just look nice, they also help absorb the sounds and dust of the city and create a calming environment, as well as helping to block the less welcome views. She recommends Stipa gigantea, a tall slender evergreen grass with tiny glistening "oat-like" flowers, and Miscanthus sinensis, which has maroon flowers that age to silver.

Vegetables and herbs can be ideal for roof gardens, adds Annie Guilfoyle, director of garden design at KLC (www.klc.co.uk). "Some, such as chard, look lovely and if you live in the city, you can grow things like chillies and aubergines that would be more difficult in the country. For something more fragrant, mint is good," she says.

Design

Roof gardens are usually small, so the furniture becomes the centre of attention, says gardener Andy Sturgeon (www.andysturgeon.com). "So make it part of your design rather than plonking it in at the end. I tend to use a combination of built-in and freestanding furniture. The former works well because lack of storage is usually an issue, and off-the-shelf furniture is good because it's more comfortable than anything you can make. Anything rattan works well because it's easy to integrate it as part of the design and it's not too bulky, heavy or lightweight. With built-in seats, we try to create storage underneath – useful for cushions and gardening equipment."

If there's room, incorporate a lounger and a table. But don't clutter the space – enjoy the wide horizons and sense of "big sky". Function is key here: what do you want to do on your roof garden? Sunbathe? Eat? Entertain? Consider modular furniture, which can easily be changed from a sunbed to a sofa, for example, as well as stackable chairs and tables.

Built-in containers can give the illusion of more space than random pots, according to Sturgeon. "They also avoid leaves and debris getting caught between them. Lighting is important because roof gardens tend to be urban and therefore are more likely to be owned by people who work and use them in the evenings. That old cliché about the garden being another room is particularly true with roof gardens."

Water features can add atmosphere, although you'll need to check load-bearing limits with a structural engineer before installation, says Amos. "Meanwhile, speakers fixed discreetly behind planting can waft gentle sounds and, if the budget stretches, a sink, fridge and worktop will make al fresco eating easy. My absolute luxury would be fittings for a hammock. I'd take sneaky glances over the top of my book at the urban skyline while weightlessly floating under the sky."

How to build up

* Firstly, employ an architect: this is not a DIY job.

* Don't worry if the area you have in mind is small, it can still provide a welcome breathing space on hot summer days, says architect Tony Brohn of Studio B3 (www.studiob3.co.uk), who recently created a dramatic zinc-and-timber-clad pod on the roof of a Victorian property.

* Don't get too excited too soon: have early discussions with the local planning authorities.

* The terrace can be partly or wholly enclosed, with retractable sliding glass panels. These should be bought from a specialist glazing company with a track record in roof terraces, advises Brohn.

* If you have a pitched roof, you can still build a terrace. Options include slicing away a section of the existing roof to create a flat space or bridging a valley roof to create an upper deck.