LEGISLATORS this week will urge the Government to shorten the maximum period of four years within which property developers must complete their flats in a bid to curb runaway property prices. A paper released to legislators ahead of Tuesday's meeting of the Legislative Council's Lands and Works panel reveals that developers are generally allowed three years to complete projects smaller than 10,000 square metres and four years for larger ones. The Government said a change was not needed. But United Democrats housing spokesman Lee Wing-tat said the period was too long and allowed developers to manipulate the market. ''In the sandwich class housing scheme, the Housing Society can finish 5,000 flats in between 24 and 30 months,'' he said. ''That means developers . . . can complete the construction any time they want and sell the flats when the property market is good.'' A government taskforce, headed by Secretary for Planning, Environment and Works Tony Eason, is working out a plan to curb property prices. The Sunday Morning Post reported last week it would include measures to control selling new flats. Mr Eason said steps to increase the supply of flats would also be taken. Lands and Works assistant secretary Trevor Keen said the Government had to strike a balance between the supply of flats and developers' problems, to ensure quality.