(Sharecast News) - Derwent London said demand for its properties was strong as rental income at the real estate investment trust increased. Lettings so far in 2019 were £17.2m on 217,000 sq ft - 6.2% higher than the estimated rental value for December 2018, the FTSE 250 company said. Vacancy rates edged down to 1.7% from 1.8%.Derwent said it had more than 1m sq ft under construction with 64% pre-let. The sites include property in London's Soho, Fitzrovia and Old Street districts. The company typically builds offices and retail space in developing areas of London.John Burns, Derwent's chief executive, said: "Demand for Derwent London's space remains strong ...These lettings combined with our financial strength put the group in a very good position to pursue new opportunities."Net debt rose by £23.8m to £980.7m in the three months to the end of March after capital spending of £48.1m. This pushed the loan-to-value ratio up to 17.5% from 17.2% at the end of 2018.Derwent shares rose 0.4% to 3,210p at 0926 BST.