Housebuilder Barratt Developments has had a weaker than expected autumn selling season, though new site openings will drive a "limited" increase in volume growth for the full year.Net private reservations per active site per week dropped to 0.45 in the four months since July 1 from 0.55 a year ago due to weaker customer sentiment. The cancellation rate rose to 15.2% from 13.9%."Conditions for the UK housing market remain challenging with constrained mortgage lending combined with weaker consumer confidence being key obstacles to significant recovery," the group said. But underlying selling prices have remained stable. A change in mix had the average selling price (ASP) on completions up 9% to £180,000 and the private ASP 12% higher at £194,000.That should drive margin growth for the year, said chief executive Mark Clare. Total completion revenues have increased by about 9% and the net increase in active sites for the financial year will be around 60."We expect changes in mix to continue to drive increases in ASP but at a slower rate in the second half, with houses increasing from 60% to around 65% of total completions for the full year," said the firm.