Estate secrets: Entranced by Lunuganga, the country house of Sri Lankan architect Geoffrey Bawa

Our stay at Lunuganga came right at the end of our journey in Sri Lanka. For more than two weeks, we'd moved slowly through the hill country in the centre of the island, riding trains, walking, tracking down waterfalls (I have a passion for waterfalls). Then we dropped off the high ground and came down to the south coast, spinning past Tangalle and Galle to Bentota. This is where the Sri Lankan architect Geoffrey Bawa finally rooted himself in 25 acres of land looking over the mirrored water of a huge lake. Over a period of 50 years, from 1948 onwards, he made the house and garden (you can't separate the two) that is Lunuganga.

We arrived in late afternoon, and it was already dusk when I first wandered down from the house to the lake. Fruit bats bigger than rooks broke away from the shadows of the palms and carried their silent blackness into the sunset. Frogs began to burble in the rice paddies. Huge frangipanis with silvered limbs stretched in a line to somewhere I could no longer see. It was a powerful introduction to the place.

From the start, said Bawa, he thought of Lunuganga as "an extension of the surroundings – a garden within a larger garden." What he'd bought was a Thirties bungalow in the middle of a rubber plantation. He had no overall plan of what he wanted to do, but two early interventions established the scale on which the garden had to develop. First, he cut through the dense plantations to open up a view north over the lake to an island in its centre. Then, in a much more complicated and ambitious move, he cleared a long wide vista which falls away from the house to the south, then rises on the other side of a dip to the highest point of the garden – Cinnamon Hill. There's an echo of the English landscape garden in this broad green ride (Bawa was a student at Cambridge and later qualified as an architect in London) but on either side of course it's hemmed in by the dense evergreenery of indigenous trees, not oak or ash or beech.

Looking out from the house, or back to the house from the top of Cinnamon Hill, that drop and rise of grass seems seamless. But actually, running across the middle, at the lowest point, there's a road leading to another property. It's brilliantly concealed. Only when you are moving through the covered passage by the Draftmen's House (built by Bawa for his assistants) does a small window show you that you are actually on a bridge crossing the road. There's no way you can approach the road directly. A dense low planting of shrubs and ferns makes an impenetrable barrier.

Having made his long vista to Cinnamon Hill, Bawa had an even more ambitious idea. If he cut the top off the hill, he reckoned, from the house, he'd see a shining sliver of the lake before his view finally ended in the misty wooded rise of a hill the other side of the water, conveniently crowned with a Buddhist stupa. He had to reduce the height of the hill by three metres before he got that silvery glimmer that he'd set his heart on.

The house, a low series of Modernist cubes, crowned with a shallow, traditionally tiled roof sits on top of a mound. The slope was accentuated by Bawa, who cut away the north side so that the ground below the terrace drops steeply to a grid of flat, square paddies. Some are planted with rice, some flooded and filled with the gorgeous blue waterlily that, of all the plants in Sri Lanka, is the thing I'd most like to be able to grow.

Just beyond the paddies, the edge of the lake is formally framed, Venetian style, by a boat terrace and, at right angles to that, a set of wide shallow steps leading down into the water. That Italian touch is a surprise, as is the grotesque horned head of Pan that leers out from the undergrowth. Between the paddies and the terrace above runs the broad walk, framed on the right by a line of statuesque frangipani, planted on square mounds.

As a garden, Lunuganga is more about the way space is manipulated than it is about plants. But the frangipani are important. Standing in a line along the broad walk, they emphasise its relative formality. Then above, planted on the terrace immediately in front of the house, they frame different views through to the water beyond. By nature frangipani are multi-stemmed rather than single-stemmed trees. On the terrace, Bawa weighted down a few of the limbs with big stones, so that the trees splayed out even more than normal. Although they are not native to Sri Lanka, they look as though they might be, with their waxy whorled flowers and leathery leaves. Unlike so many sub-tropical trees, they are not evergreen and are at their best, I think, when the leaves have dropped and the branches with their stubby ends, stand stiffly in silhouette. Bawa planted the terrace trees in the Fifties, which perhaps was also when he built the retaining parapet of the terrace – not in a straight line, not in a simple curve but as an intriguing series of curlicues and brackets.

The garden is most complex close to the house, as is usually the way. Steps, fitted at unusual angles in between retaining walls, move between buildings and small courtyards. To the east, you arrive at the superb galleried garden room that Bawa used as his studio, using ancient blue painted doors and shutters rescued from demolition sites in Colombo. The inside spaces are wide, airy and high-ceilinged with double doors standing open, catching the breeze. The vista, Italian fashion, runs right through the middle of the main house, from Cinnamon Hill on one side, to the lake on the other.

Bawa said that, for him, the most beautiful thing about Lunuganga was "the effect of sunlight filtering through the leaves". I thought sitting in the inside-outside space of the verandah was pretty good, darkness falling, the shadows of palms flickering on the ceiling, a Lion beer to hand and the prospect of a brilliant Sri Lankan curry and rice ahead.

Lunuganga, now owned by the Bawa Trust, is 60 miles south of Colombo at Dedduwa, Bentota, Sri Lanka, +94 34 428 7056. For more information go to the website at lunuganga.com or you can e-mail lunuganga@sltnet.lk. There is only one guest room in the main house. You can also stay in one of the five Bawa buildings scattered through the garden. Room rates vary according to the time of year. In high season expect to pay $391 for the guest room in the main house, $426 for the gallery suite. Prices include breakfast. The garden can be visited by appointment every day (11am-3pm) except public holidays, admission 1,250 rupees.

For a taster, read 'Bawa: the Sri Lankan Gardens' by David Robson (Thames & Hudson)