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Room with a view: How a central London roof top became a spectacular garden in the sky

Mark Baxter and his partner needed a garden designer who could turn their Bermondsey Street roof garden into a sleek, sharp space sympathetic to the rest of the flat. Enter Charlotte Rowe...

I'm sitting in the sun outside the Caphe House Vietnamese café in Bermondsey Street, just south of London Bridge station and the temperature is nudging 33C. This might not happen very often in British summers, but when it does, you realise how vital a bit of outside space becomes for people living in cities. It was number one priority for 43-year-old Mark Baxter and his partner Robert Bready when they first started looking for a property in this area.

"Bermondsey hadn't really happened as a place to live when we first came here," he explained. The building he's in was still a shell, but when they climbed to the apartment being made at the top, they were instantly hooked. From the flat roof there were stupendous views west, all the way round from Tower Bridge to Big Ben. The Shard hadn't been built then, but now its glittering face reflects the morning light on to Mark's terrace, so he's come to think of it as rather a good thing.

In the first Bermondsey years, the summers were so awful that Mark and his partner didn't use the roof much. "We had a table and chairs up there, but they were kind of lonely, marooned in all that space." But then, having done up the rest of the flat (they are both in the fashion business and the look of things matters very much) they decided boldly to build another room on the flat roof.

Enter Charlotte Rowe, whom Mark found by searching on the internet for a garden designer who could work in a sleek, sharp style sympathetic to the rest of the flat and who understood the special complications of roof gardens. "And they are complicated," said Charlotte as the three of us stood out in the elegant space she has created, almost 8m long by 3.5m wide.

Before you even get round to the design itself, you have to think about "future-proofing" (Mark's phrase) the existing roof. Where are the load-bearing joists? Where is the wind likely to be coming from? What are the planning issues? How are you going to get your building and planting materials up five storeys? What plants are likely to survive in this exposed, windy, drought-prone habitat? How will they be watered and fed?

Crucially, the space left over from the building of the new room was well-proportioned, almost twice as long as it was wide. Charlotte came up with a design that divided it into two, a seating area immediately outside the sliding glass wall of the new sitting room and a dining area beyond. Brilliantly, she also found a way of incorporating a rather wasted bit of roof on the opposite side of the room, a potentially grim little area alongside the stairs leading up to the roof.

Viewed from the stairs, this narrow space now has the enclosed, mesmerising, self-contained air of an aquarium. Two huge (800mm) black cylinders are planted with clipped choisya 'Aztec Pearl' and four similar but smaller (600m) pots are planted with multi-stemmed birch, which do surprisingly well on roofs. On the wall behind are two tall panels of dark-grey mirror glass. "I wouldn't normally do mirror," says Charlotte. "It can be kitsch. But Mark wanted them and these actually work very well." As she says, you have to listen to your clients, as well as guide them. But having had a former life as a PR for Ken Livingstone's GLC, she's very good at managing expectations.

The roof garden is floored in slats of Western red cedar, with much narrower slats of the same wood framing the three-sided sitting area, tucked into the right-hand corner. The planting is fairly minimal, dominated by three elegant olive trees set in a raised bed that, together with one arm of the seating area, provides a screen between this space and the dining area beyond.

Running along in front of the olives, like a long headrest, is a neatly clipped rosemary hedge. Three balls of choisya 'Aztec Pearl' sit at the corners of the built-in seats. This, together with four box balls (40cm across) ranged along the back boundary of the dining area, is the limit of the planting. The box balls work particularly well because they are set in handsome chocolate-brown stoneware pots, only 360mm in diameter but a metre tall.

Mark (who says he's "quite fussy") stipulated only architectural plants, evergreen, easy-care. For him they are set-dressing; he's not particularly interested in them per se. The roof garden is maintained by a garden company and the pots watered by an automatic irrigation system, with the controls hidden under the lid of one of the seats.

He's not too sure about the choisya balls in the seating area; he thinks they have got "a bit scraggly" and aren't architectural enough. He doesn't want any of thef spiky things that might survive up here, so Charlotte carefully goes through the rather limited range of plants that will fit his brief: Pittosporum tobira, Viburnum tinus, Osmanthus x burkwoodii, mops of Pinus mugo. My bet is that if he does decide to ditch the clipped choisya, he'll go for more box balls, as the ones in the tall pots seem to be thriving.

I'm impressed by the careful attention to detail in the design of the roof garden. The olives are actually growing in a big galvanised trough, disguised by low walls of marine plywood with a rough lime-like finish and panels of narrow cedar wood slats. Where they meet at the corner, the slats are mitred, not just butted up against each other. The area enclosed by the seats is defined by decking laid at right angles to the rest of the floor. The screen that shields some ugliness to the north is made of narrow horizontal slats of the same cedar, the space between them increasing as the screen gets higher. It stops it feeling like an imprisoning boundary.

Square metre for square metre, a roof garden will always be more expensive to install than a regular one. The access will be more complicated, so the labour costs will be higher. You have to bring in all the soil. You will probably have to spend money on the underpinning (sealing the roof, laying waterproof membranes, installing drainage) before you even start on the garden itself. For a small roof terrace of 25-30 square metres (Mark's is about 28sq m) laid with decking or stone, plus containers, plants and soil, trellis, lighting and irrigation, you'd probably end up spending £35-£45,000 plus design fees. The view is free.

Charlotte Rowe Garden Design is at 118 Blythe Road, Brook Green, London W14 0HD, 020-7602 0660, charlotterowe.com