Error occurred in deployment step 'Install app for SharePoint': Sideloading of apps is not enabled on this site

If you are starting to develop Apps for SharePoint or Project Server and you encounter the problem where you get the following error message:

 Error     1     Error occurred in deployment step 'Install app for SharePoint': Sideloading of apps is not enabled on this site.             0     0     SharePointApp_01  

It is usually because the site you are developing against is not a SharePoint development site. The easiest way to solve the problem is of cause to create a new SharePoint Development site.

However this is not always possible and you might have a lot of content on your site you want to test against. Furthermore if you are creating a Project Server App I have not found a way to create a Project Server Development site.

Convert SharePoint (and Project Server) site into a Development site
This can simply be done through Powershell. To be able to run Powershell agains online you will need to download SharePoint Online Management Shell.

The Powershell script you need to run is:

 $programFiles = [environment]::getfolderpath("programfiles")  
 add-type -Path $programFiles'\SharePoint Online Management Shell\Microsoft.Online.SharePoint.PowerShell\Microsoft.SharePoint.Client.dll'  
 $siteurl = 'https://MyTenant.sharepoint.com/sites/pwa'  
 $user = 'admin@MyTenant.onmimcrosoft.com'  
 $pass = ConvertTo-SecureString -String 'password123' -AsPlainText -Force  
 [Microsoft.SharePoint.Client.ClientContext]$cc = New-Object Microsoft.SharePoint.Client.ClientContext($siteurl)  
 [Microsoft.SharePoint.Client.SharePointOnlineCredentials]$spocreds = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($user, $pass)  
 $cc.Credentials = $spocreds  
 $site = $cc.Site;  
 $featureDevSiteUid = new-object System.Guid "AE3A1339-61F5-4f8f-81A7-ABD2DA956A7D"  
 $site.Features.Add($featureDevSiteUid, $true, [Microsoft.SharePoint.Client.FeatureDefinitionScope]::None);  
 $cc.ExecuteQuery();