House & garden: How one London home seamlessly blends outside and in

I arrived early for my appointment with Vivien Fowler at 5 Garden Close in Putney, south-west London, so had plenty of time to poke around the surrounding streets, looking at trees, peering into front gardens, trying as always to work out how all this stuff happened, and why. To me, a country person, it's like being abroad. The bonus is that in this country, I am slightly better equipped to unravel the clues.

I was in a strange little enclave of roads and cul-de-sacs marooned on Putney Heath, the bit of it that lies north of the Kingston bypass: Heathview Gardens, Bristol Gardens, Portsmouth Road. That gave a clue. This must once have been the old road south out of London, before the thundering A3 took its place. A couple of fantastic Art Deco houses fixed one phase of development – whitewashed walls in sinuous curves, Crittall windows, sleeping platforms above built-in garages. An integrated garage. What a show-off that would have been in the Thirties, when film stars Cicely Courtneidge and Jack Hulbert lived here.

Heathview Gardens must surely be earlier, with magnificent great trees – sweet chestnut, lime, copper beech, holly and bay. It has the best pavements I've ever seen, mostly still original work. They are unusually broad, which gives a comfortable feeling to the curving road and allows the use of several different materials. At the street edges are hefty granite kerbs. Inside those, a band of stone cobble, then a broad section of paved brick with another band of stone cobble meeting an edging of rope work tiles. These retain narrow outside borders, which the residents must once have been expected to plant up. In one or two places crass new wooden fences have encroached on and obliterated this nice final flourish. Planning authorities interest themselves in the height of boundaries, but in neighbourhoods as intricately detailed as this, perhaps they ought to interest themselves in the style as well. It matters.

So I was full of questions when I finally made my way to Garden Close. Vivien Fowler explained that the whole area had once been Pitt the Younger's country home. Their own quarter-acre plot, developed in the late Eighties, was part of the old walled kitchen garden. Her husband, Tom Jestico, is an architect, and when they bought the plot, he designed a glass cube to sit in the middle of it. The walls of the garden become in effect the walls of the house. There is little distinction between inside and outside space. The effect is ravishing.

You come in to their place through a narrow gate which breaches the tall brick wall on the west side of the kitchen garden. Immediately ahead is a terrific set piece, faintly Oriental in style, with the low glass house slightly screened by lines of black-stemmed bamboos on left and right. The first thing I noticed is how well the bamboos are managed, the growths thinned out, all the dead stuff cleared away, the leaves stripped up to a certain point on the stems. They looked superb. A wide stone path leads up to the house, with grass bending round on either side. Ahead, the path breaks into rounded stepping stones before making what seems to be a bridge over a wideish moat. You have to cross this before you hit the wooden deck that runs round the whole house.

Although the overall effect is very calm, very soothing, there is a fantastic amount of detail to absorb. From the deck, you have time to take it all in, looking as it were from the house out, rather than from the garden in. If you are used to solid walls in a house, this confusion of outside and inside is wonderfully liberating. The path continues as a central axis through the house, meeting at right angles a longer corridor which runs all the way through the house from north to south. Smooth white walls, smooth white cupboards, few doors, masses of light – you feel curiously that a load has been lifted from your shoulders. And wherever you go in the house, the garden looks back in at you, floor to ceiling.

From the deck, with huge pots of acer, olive and camellia alongside, you can admire the catalpa with leaves as big as placemats, the birches planted in the grass and the amazing sheen on the leaves of the fatsias, set behind the black bamboo screen. To the south is a huge old apple tree, cleverly limbed up and cleaned from underneath, so it looks like a piece of sculpture. Underneath it, hostas and Japanese anemones crowd the space. On the north the house looks out on to a simple sea of ivy and beyond, the fine old brick wall of the original garden. It's a wonderful colour, pink and slightly mauve, not the usual cream and yellow of London stock brick. Clematis armandii flings its arms over a stretch of the wall, but, quite rightly, plenty of it is left bare.

The south and east sides of the house face the most exuberant parts of the garden, the swathes of lavender that once filled the bed on the south side replaced now by a medley of hot colours: red, orange and purple, with red hot pokers, Crocosmia latifolia 'Lucifer', nasturtiums, alliums, poppies, Cirsium rivulare. All down this side too are the old fruit trees of the original kitchen garden. The apple I'd seen on the lawn was just the beginning of a thread that still stitches the site firmly back into its past. I liked that, especially in conjunction with the clean, modern architecture.

At the back of the house is a vegetable garden planted up in a bed still contained by its original box hedge. Most of the space is taken up with salad crops, but there are plenty of herbs alongside and behind, the handsome Prince of Wales plumes of cavolo nero, the Italian black kale.

It's rare to find a house and garden conceived as a single unit like this. Rare too, to find plants used in such a quiet way. Apart from the sunny border on the south side, plants are mostly used in biggish swathes: sweeps of rhododendron, valued for its evergreeness as much as its flower, grey-leaved hebe clipped into undulating hummocks, big clumps of sedum, stands of bamboo, rivers of lavender, carpets of ivy. "I like structure," says Fowler, who has spent her life as a designer. "Once you've got the concept, the planting just falls in round it." Both garden and house (look out for an extraordinary Charles Eames seat, like a Henry Moore sculpture) are open tomorrow (11am-5pm) at 5 Garden Close, London SW15 3TH, admission £2.50.