OFFICE rents continued their relentless rise in July, with prime space in the main business districts recording an average increase of 13.9 per cent since January. The monthly Hong Kong Office Market Review from City Property Consultants found rent for space in Central and Admiralty soared a further 2.8 per cent to $52.93 per square foot net last month alone. The average cost of renting office space is now $48.28 per sq ft net, which shows it increased 2.9 per cent last month. Vacancies across the board stayed low at 1.8 per cent. City Property Consultants' director Hugo Poon said because the supply of new office space in these districts was limited, there was little chance the rental rise would stop. The report says office rents in Wan Chai and Causeway Bay rose by 2.9 per cent to $40.42 per sq ft, compared to $39.28 per sq ft in June. The districts showed a dramatic increase of 15.7 per cent this year. The vacancy rate moved up to 2.3 per cent last month, compared to two per cent in June, which was a record. The amount of vacant space in Wan Chai and Causeway Bay has remained about two per cent for several months, but in Central fell to 1.4 per cent in May, where it has stayed. The reports says rents for the main office towers changed only slightly in July, but continued to rise. Exchange Plaza, which remained the most expensive building, moved up $1 to $65 per sq ft. Rents in Tsim Sha Tsui rose by an average of 0.44 per cent in July, according to the monthly office market review by Collins' surveyors.