i m using sharepoint webservice admin.asmx to create a site collection in my webapp
using following method
adminService.CreateSite(fullUrl, siteName, string.Empty, LCID, SITE_TEMPLATE, loginName, displayName, string.Empty, string.Empty, string.Empty);
adminService is object of this service
Make sure that you are giving the sharepoint administrator credential to the webservice before calling the adminService.CreateSite…