I have the following convention for most of my projects:
/src /Solution.sln /SolutionFolder /Project1 /Project2 /etc.. /lib
In addition to Shane Kms answer, if you've activated Nuget Package Restore, you edit the NuGet.config located in the .nuget-folder as follows:
..\..\ExtLibs\Packages
Notice the extra "..\", as it backtracks from the .nuget-folder and not the solution folder.