YOU hear the place before you see it. A pair of boisterous roosters crowing, pigeons cooing and mynahs perfecting their speech in cages under the thick leafy canopy. This is neither a poultry farm nor bird house, but the rooftop of a 23-storey high-rise at the heart of Yau Ma Tei. In fact, this sky-high Chinese roof terrace seems anything but what it really is. At ground level, it looks like a bird haven where flocks of pigeons circle and rest on the abundant greenery which spills over the roof. But a closer look inside this secret garden reveals rabbits hopping about in a metal cage and cats stretching lethargically in the sun on deck chairs. You can hear water bubbling in a tank of koi and smell the pungent odour of poultry droppings. As an old Chinese saying goes, an inch of gold can buy a foot of soil. So why, in Hong Kong where the tiniest plot of land can raise a king's ransom, waste a roof which can be turned into a small poultry farm, green house, swimming pool, playground, barbecue area, tennis court, and even (in the old days) classrooms and animal training grounds? The people who live in the Yau Ma Tei block off Nathan Road do not seem bothered about their neighbours' life on the roof. None of them finds the constant cooing, crowing and twittering a nuisance. ''Well, I have no idea what's up at the top but I don't care either,'' said a young woman who lives on the floor below. Across the harbour, children run about on the North Point Methodist Church Kindergarten's rooftop playground. There is room for more than a dozen pupils slip down slides and clamber over climbing frames. But it was not intended to be this way when the school, tucked away between housing blocks on a quiet hillside, was built some 30 years ago. ''Someone realised that the children did not have enough space to play, or run around, on,'' said the kindergarten's headmistress Angela Luk. ''So in 1986, we built a new wing and renovated its roof. ''It was cheaper to build a playground on a roof than on an extra piece of land. What's more, we don't have to worry about falling objects as our roof is taller than surrounding buildings.'' But not all rooftop structures were built with a particular purpose. Remember that airline ad showing a man relaxing on an air bed on a circular rooftop swimming pool overlooking Hong Kong island? That was the top of the Hopewell Centre on Queen's Road East. ''It's not really a swimming pool,'' said Hopewell property director Julia Lui. ''It's just a pool of water. No one swims there. Frankly, we don't know why a pool was built there.'' Rumour has it that a fung shui master thought the building looked too much like a candle and would be doomed to catch fire. Placing a pool on top would never let the wick start to burn. ''No, I haven't heard anything from that angle,'' Mrs Lui said. ''We know the pool was part of the overall construction of Hopewell Centre and that it was quite costly to build.'' To design, construct and maintain a rooftop terrace is, indeed, a costly affair. But if you have several hundred thousand dollars to spare, to stroll around a 1,800-sq-ft classical Chinese garden outside your penthouse suite is sheer luxury, as businessman Kan Hing-fook will tell you. The 74-year-old planted six orchards, installed a mini goldfish pond and constructed a small kiosk overlooking the whole of Central on his Kennedy Road penthouse. ''Someone made me an offer to buy this place for $25 million. I turned it down. Though I also have a big flat on the Peak, it is nice to come down here and relax every now and then,'' he says. As Hong Kong land values continue to soar, are penthouses the most sought after property investments since buyers, who only have to pay a little extra, get both the residential unit and a roof? ''Not really, '' says a leading estate agent in Hong Kong. ''In general, these units are not wanted at all. Perhaps 10 out of 100 will buy an ordinary penthouse. It all depends on the developer and property. A typical penthouse tends to have a roof that leaks.''