A models already exists. They are situated in another project. Where I should put DataAnotations in that project or my one? Should I use partial classes? I would like to put DataAnatation because I want javascript validation to work on client.
You can't use partial classes across assemblies, so that option is out.
You can create DTOs (data transfer objects) that are copies of the ones in the other assembly, annotate them and map.
For easy mapping you can use a tool like auto mapper. If the property names match, it will essentially do all the work for you.