WHEN Mr Cheung leaves his 400-square-foot Choi Hung public housing estate flat in the morning for the MTR trip to his office, he often sees his neighbour getting into a BMW. Mr Cheung describes his neighbour as ''a rich tenant'', but the BMW driver is just one of many seemingly prosperous inhabitants of what is regarded as a ''well-off'' estate, where most of the flats are air-conditioned and have washing machines and video recorders. The three-storey car park underneath the block is dotted with Mercedes-Benz, BMWs, Toyotas and Mazdas, and during Chinese festivals limousines stop on the road outside to drop off families returning home for a reunion. At a North Point public housing estate, tenant Mr Ng said: ''Those who've lived here for 10 years or more are much better off now. It's reasonable for them to pay more.'' Mr Ng, 25, who earns $10,000 a month, looked at a Mercedes in the car park and added: ''Most people are able to pay, but they don't want to lose the benefits they are enjoying.'' Such public housing tenants are the envy of thousands in Hongkong who have no option but to pay exorbitant rents on private flats. However, Mr Cheung's ''rich tenants'' are the very people who are fighting the Housing Authority's controversial plans to introduce a double rent policy, officially called the Housing Subsidy Policy. Under the recently revised policy, tenants who have lived in public flats for more than 10 years are required to pay double the normal rent if their monthly income is more than treble the limit allowed for public housing. Tenants with income amounting totwice the subsidy limit pay 50 per cent more. Following protests, legislators passed a motion last December in favour of scrapping the policy, but the Housing Authority claims the motion is not binding. About 2.5 million people live in public housing and more than 250,000 of them have incomes above the subsidy limit - $4,600 a month for a single person and $20,000 a month for a family of 10 - yet only 62,500 tenants pay double rent. One of them is Mr Kenneth Tai, a financial adviser, who has just returned from a holiday with his family. He lives in a well-furnished 400-sq ft flat in a North Point estate with his wife, parents and two children. Mr Tai said he and his wife earned $23,000 a month. But he is opposed to paying more for his 30-year-old flat, which he described as noisy and dirty. He said the $2,249 monthly rent was too high. ''If it is a subsidy scheme, the basic rationale should not be to make a profit. ''Apart from the double rent, there is a 21 per cent increase in rent every two years. I think the best thing is to scrap the double rent policy.'' Mr Andy Wong also pays double rent for his flat on the North Point estate. He objects to paying double rent for the sea-view flat even though his family earns more than $22,000 a month. He said the $2,200 rent was ''only reasonable when you compare it to renting a private flat''. ''Even if they didn't force us to pay double rent, the normal rent is adjusted every two years,'' he said. ''If we were really rich tenants, we would have moved out to a bigger and better private flat long ago.'' Meanwhile, a housing watchdog group, The Hongkong People's Council on Public Housing Policy, yesterday criticised the Housing Authority for a lack of public housing, even though 45,000 flats will come on stream in this financial year compared to 39,000 last year. About 14,000 of these flats will be allocated to those on the waiting list, which the group claims comprises 176,000 people; the Housing Authority says it is 68,000. The watchdog body claims 50,000 new flats would have to be built every year to meet demand.