The need for metals during World War II led to a boom in the copper industry which, accompanied by a 1941 excess-profits tax ordered by the UK, brought increased revenue to the government. The boom also led to a substantial increase in urbanisation. Lusaka's official population (which excluded much of the African population) rose from around 2,000 in 1931 to almost 19,000 in 1946, with a 15 per cent annual growth rate.   In 1948, the government passed the African Housing Ordinance, which authorised permanent residential suburbs for Africans, including married couples. Employers in the city were also required to pay for their workers' housing. The city authorities founded the African Housing Board, which built the new suburbs of New Chilenje and Matero. Lusaka's African workforce, which remained relatively unskilled, did not benefit as much as that of the Copperbelt, where a shortage of skilled mining labour had led to improved pay. In 1952, a development plan for Lusaka gained statutory approval for the first time – earlier proposals such as those by Adshead and Bowling had never been legally sanctioned. This plan envisaged much more territory for the African suburbs, but only around one-third of these were ever built.