ANYONE who claims they can help in selling or buying flats for others will be required by law to declare their interest in the transaction and disclose as much as they know about the property to the buyer and seller. The rule is part of a system being worked out by the Government to regulate the 2,500 property agents in Hong Kong. The move follows growing concern over sales tactics, malpractice and over-charging in the industry. The number of complaints about property agents reported to the Consumer Council rose from 80 in 1988 to 248 in 1991. Calls for a regulatory system reached a high when three estate agents were convicted last year of tricking clients into selling their properties below market levels and then reselling them at a higher price. Estate agents exercise self-regulation at present and any person can open a shop with a business registration. Leo Kwan Wing-wah, Deputy Secretary (Housing) with the Planning, Lands and Environment Branch, said: ''One of our difficulties is that there is no legal definition of a property agent. And this gives rise to grey areas.'' But if the proposed regulatory system is approved, anyone who claims he can help others in property transactions will be defined as an estate agent and will have to follow a set of rules. A legal framework will be set up to require an agent, as a middleman in the transaction, to tell his clients in as much detail as possible what he knows about the property, Mr Kwan said. An information sheet could be introduced, stating the things an agent is required to provide in the transaction. This will include the age of the flat, the floor area, whether it is for domestic or commercial use and the expiry date of the land lease. The Government plans to ask estate agents to use a standardised provisional sales and purchase agreement to stop confusion caused by the use of different agreements or forms. Mr Kwan said the Government understood the difficulties in obtaining information about old buildings and was looking at ways to solve the problem. He heads a working group to find ways to regulate estate agents. The group comprises representatives from the estate agent industry and legal profession. ''We aim to work out a licensing system where the requirements will not be so tight as to drive out too many of our existing estate agents. We must have enough agents in the industry to serve the buyers and sellers,'' Mr Kwan said. The working group is expected to release a report in July.