The City Council decided Monday to hire an independent auditor to analyze a staff recommendation to buy or build a new city hall. The city's lease of office space at the Pala Building, for about $400,000 a year, expires in 2001. According to the staff report, buying the Pala Building for about $10 million or constructing a new city hall next to the Mission Viejo Library would save the city money in the long run and would accommodate increases in staff. Residents voted against a city hall proposal in 1992. An advisory vote would be required if the council decides to pursue the issue again.