How specific does a "Work Package" in a grant proposal need to be?
Try to look at it from the view of a reviewer. You'll have a limited amount of time to estimate whether 


the research will have a good chance of success (knowledge generation is success, so negative results are allowed!) 
the applicants know the field well enough to work on the topic without stumbling about too many obstacles (e.g. you should say which methods you are using and why)
the work load is managable
the project is well managed.


Every reviewer knows, that you can not predict the future. If you could detail the research you want to do in three years, it would not have been research in the first three years! But you should make clear you know of the risks of your proposal and demonstrante, how you will handle those uncertenties.