Self storage firm Big Yellow said the subdued economy, higher interest costs and VAT on storage continues to weigh on earnings, but it still managed to post a strong rise in adjusted profit before tax.Adjusted pre-tax profit climbed to £13.9m in the six months ended September 30th from £11.6m a year earlier. Store revenue for the six months rose 11.5% from the year before. Like-for-like (LFL) revenue per available foot (REVPAF) was £21.28 for the six months, up 10.1% from the year before.Store revenue during the second quarter increased by 12% to £18.6m. Chairman Nicholas Vetch said: "This is a strong revenue and earnings performance in a challenging environment for consumer facing businesses such as ours." "As previously indicated, the impact of higher interest costs and the imposition of VAT on storage will create a drag on the pace of earnings growth in the short term. Additionally, we believe the economy will remain subdued for some years to come." However, Big Yellow believes the strength of its brand, growing market awareness, and its concentration of stores in London and the South East will help it overcome these challenges.An interim property income dividend of 5p per share has been declared, up from 4.5p before. Net debt reduced to £269m from £273.9m at the end of March 31st 2012.CJ