We recently worked with valtech to do the some analysis and development work for us. The model we followed was:
1. Initial analysis work understanding of personas, user stories etc… in house
2. Bring the third party to clearly understand, estimate, and define the acceptance criteria. This also included the definition of done for each iteration and what percentage of technical debt can be carried over.
3. Third party will do the development work.
4. The work is analyzed and accepted or rejected in house…based on acceptance criteria and definition of done.
5. Third party is paid accordingly.