Expanding the size of your home is a balancing act between different priorities- cost and budget, the needs of your family, the best layout and structural design for the addition, and of course fitting in seamlessly with your existing home and neighborhood.
Typically local municipal zoning regulations are in place for a variety of reasons, so an understanding of the purpose of the regulation could help you determine if a green roof can help your home stay within compliance with the addition.I would say the first step would be to contact your local planning commission or zoning office about your property to make sure you have all the information you need about your requirements and what would be considered as an exception. A green roof could certainly help with permeable space and help improve storm water quality and quantity; as well as reducing heat island effects. Open space requirements can sometimes be about storm water quantity and quality, but not always.
Another motivation behind open space requirements is cosmetic; perhaps the ordinance is in place to keep visual appeal and keep new development in harmony with existing homes. Established neighborhoods sometime have open space requirements to keep adjoining neighbors from feeling "closed in" if adjacent properties have large structures that take up most of the lot.
Another important step is to work with your contractor to confirm that the green roof you have in mind is in compliance with your local building codes, and if any items in your design might need to be altered.
Also, it is a good idea to check on the inspections process for any green implementation that may not be typical in residential in advance. For example, will you need to provide documentation showing the structure can support a green roof? Will the inspector need a letter from the green roof subcontractor confirming that the material is waterproof and resistant to root penetration? I have found that a meeting in advance can be valuable.
Last but not least, you may have a local neighborhood association with design review that would need to check in with as well. I hope this helps. I have found that local municipal authorities are would like to learn about and accomodate green building practices, and always appreciate a local homeowner or contractor who contacts them in advance of a project's implementation.