PRICES for Grade B office space in Wan Chai and Causeway Bay have risen 40 per cent in the past 12 months as smaller companies try to escape high rentals in Central and Tsim Sha Tsui, according to property agency Colliers Jardine. Associate director Raymond Fung said Grade B offices in Wan Chai and Causeway Bay were now selling for more than $5,000 a square foot, against the $3,000 average at the end of the first quarter of last year. The greatest rise was in the middle of 1992. Mr Fung said prices were also being pushed up by tenants already in these two areas wanting to move to newer, more upmarket buildings offering individual air conditioners, private toilets and other extra facilities. Figures from the Government's Rating and Valuation Department indicate that the supply of new Grade B office space is growing much slower than of Grade A, particularly in Wan Chai and Causeway Bay, which have 204,000 sq ft of Grade A office space. According to Mr Fung, demand is getting particularly tight for Grade B units of between 1,000 and 1,500 sq ft. Smaller businesses are keen to buy space to insulate themselves against rising rentals, but Sino Plaza and Causeway Bay Plaza II are for rent only. Colliers Jardine has been appointed sales agent for a number of Grade B developments in the Wan Chai area. It handled the sale of the Centripech Industries' development in Causeway Bay - a 22-storey Grade B development, which, according to Colliers Jardine, was sold out within a week of being put on the market last autumn - and has been appointed sole sales agent for the Chung Wai Commercial Building on Lockhart Road in Wan Chai. The units sold last autumn started at $3,600 a sq ft. The minimum price for low floors in the Chung Wai building, which will be completed in the middle of this year, will be $4,175, and Grade B space on the upper floors of the Chinaweal Centre is sellingfor $5,800 a sq ft.