THE Hui family is renowned in business circles for its ultra-conservative approach to property development. But when you can be so sure you are on to a good thing, why rush? Their redevelopment of Shell House and neighbouring Hartlane House perfectly illustrates the old saying ''patience is a virtue''. The family has owned both buildings in Queen's Road Central for years through its company Central Development. The temptation to redevelop them to make a fast buck must have been enormous. Shell House and Hartlane House are currently well below their permitted maximum plot ratio, only 17 storeys high - small by Hong Kong standards. Had the Huis redeveloped earlier they would have reaped the rewards of higher overall rental income from increased rentable space all these years. But they would have had to compete for tenants with a whole string of other buildings in the area coming on stream at the same time. By holding back the Huis have timed their redevelopment perfectly. When their new development comes on-stream in late 1995 or early 1996, it will hold a monopoly on the supply of new large Grade-A office floors in Central. Finding tenants will be easy. The Huis will virtually have a free rein to set whatever rents they like and hold tenants on long contracts. Demand should be enormous and supply in Hong Kong's core business district desperately scarce due to the severe shortage of developable space until the Central-Wan Chai reclamation is completed. Already less than one per cent of office space in Central and Admiralty is vacant and more and more overseas companies are setting up in Hong Kong by the day. While many tenants have the choice of seeking cheaper office accommodation elsewhere in Hong Kong, most of those providing financial services do not enjoy such freedom. They tend to be tied to the middle of Hong Kong's financial hub, close to such key institutions as the Hong Kong Stock Exchange and Futures Exchange. Tenants will have to like it or lump it. Meanwhile, Central Development will be sitting pretty.