As per the title of the this post, here is a very quick code sample demonstrating how to check if a sub web already exists using the .NET CSOM in SharePoint 2013 (I imagine this should work on 2010 also):

bool WebExists(string siteUrl, string webUrl) { //connect to the root site using (ClientContext context = new ClientContext(siteUrl)) { // load up the root web object but only // specifying the sub webs property to avoid // unneeded network traffic var web = context.Web; context.Load(web, w => w.Webs); context.ExecuteQuery(); // use a simple linq query to get any sub webs with the URL we want to check var subWeb = (from w in web.Webs where w.Url == webUrl select w).SingleOrDefault(); if (subWeb != null) { // if found true return true; } } // default to false... return false; }

You would call this as follows:

var siteExists = WebExists("http://host/sites/site/", "http://host/sites/site/subsite");