Berkeley Group, the London and south-east England focused housebuilder, has had a "strong" first half and thinks it can beat original expectations for the full-year.Profit before tax for the six months ended 31 October grew 18.5% to £61.6m as revenue jumped 16% to £336.2m. Net asset value (NAV) per share was up almost 5% to 667.6p.The company has acquired 13 new sites with 2,500 plots since the start of the year, including prime London sites in Westminster and on Hammersmith Embankment.It's added £263m of future gross margin in the period, increasing the value of the firm's land bank by 12.9% since 30 April to £2.3bn. Sales reservations have risen by about 20% versus 2009/10 as new planning consents on over 30 sites last year has increased the number of sites from which the company is selling by 30%. Private reservations per site have remained at similar levels, but affordable housing has experienced a decline, as expected."Overall, this strong performance provides the board with confidence that Berkeley can outperform management's original expectations for the current year and is well placed for the following year," managing director Rob Perrins said.