At least two of the building sites touted by the government as the answer to easing the city's housing problems are smaller than a tennis court, a conservation group has discovered.

Financial Secretary John Tsang Chun-wah said in last week's budget speech that the government would be selling off more "compact" sites to help increase the supply of flats.

Green Sense found the smallest of the 21 mini sites is at Sik On Street, Wan Chai, and measures 208 square metres. A tennis court is about 260 square metres.

The group's president Roy Tam Hoi-pong also highlighted two sites in Shanghai Street, Mong Kok. "The district is known for its overcrowding and air pollution," he said. "We do not see why the government has chosen to assign more land for building flats in the area."

One site measures 252 square metres and the other measures 611 square metres. Tam said: "The two sites are so small and cannot contribute much to housing supply. We believe they should be zoned for community use, like parks, as a buffer area for residents living in Mong Kok."

On his blog yesterday, Secretary for Development Paul Chan Mo-po said selling compact sites would encourage small developers to bid, boosting competition.