Suzyann,
Personally, I would invite each builder to submit a cost for the works specified in the drawings and then narrow it down to 2 - based on price/references/local recommendations.
Assuming they come onsite to view location/access/etc, I would ask them to also quote for both ripping out and disposal of the bathroom & banisters and the installation of new (also assuming you'd choose and purchase your own suite and banister yourself).
You could of course have the architect add the list of extras to the original tender but imo, it's always good idea to meet any tradesman face to face before employing them.
Bren