How to Create Sub Site and Site Collection in SharePoint by using object model

New SharePoint Site

When you create a new SharePoint site there are two options:
   1. A new Site Collection - use the Add method of the SPWebCollection class.
   2. A new Sub Site - use the Add method of the Webs collection of the SPWeb class.

The following examples create a new Site Collection and a new Sub Site that is based on information gathered from the request form and stored in the Site Request List.

Create Sub Site

private string CreateSubSite(SPListItem item) 
{ 
   SPWeb newWeb = null; 
   string url = string.Empty; 
   try 
   { 
      SPSecurity.RunWithElevatedPrivileges(delegate()  
     { 
        using (SPSite site = new SPSite(form.HostUrl)) 
       { 
          site.AllowUnsafeUpdates = true; 
          using (SPWeb web = site.OpenWeb ()) 
         { 
              string relativeUrl = item["ID"].ToString().PadLeft(6, '0'); 
             web.AllowUnsafeUpdates = true; 
            newWeb = web.Webs.Add(relativeUrl, 
              item["Title"].ToString(), 
              item["Description"].ToString(), 
              1033, 
              item["Template"].ToString(), 
              true, false); 
           url = newWeb.Url; 
          newWeb.Dispose(); 
         } 
       } 
    }); 
} 
catch (Exception ex) 
{
 // handle exception 
} finally
{ 
     if (newWeb == null) 
        { 
         newWeb.Dispose(); 
        } 
      } return url; 
}

Create Site Collection

private static string CreateSiteCollection(SPListItem item) 
{ 
  SPSite site = null; 
  SPWeb newWeb = null; 
  string url = string.Empty; 
  try { 
     SPSecurity.RunWithElevatedPrivileges(delegate() 
       { 
         SPWebApplication webApplication = null; 
         webApplication = SPWebApplication.Lookup(new Uri (YourHostUrl)); 
         string relativeUrl = ""; 
         relativeUrl = item["ID"].ToString().PadLeft(6, '0'); 
         site = webApplication.Sites.Add(YourHostUrl + "/" + relativeUrl, 
                                         item ["Title"].ToString (), 
                                         item["Description"].ToString (), 
                                         1033, 
                                         item["Template"].ToString (), 
                                         siteAdministratorLoginName, 
                                         siteAdministratorName, 
           ;                             siteAdministratorEmail); 
         newWeb = site.OpenWeb(); 
         url = newWeb.Url; 
         web.Dispose (); 
         site.Dispose (); 
       }); 
      }  
  catch (Exception ex) 
   { 
     // handle exception
    } 
   finally 
   {
      if (newWeb!= null) 
       { 
         newWeb.Dispose(); 
       } 
      if (site != null)
       { 
         site.Dispose(); 
        } 
    } 
 return url; 
}