Once you’ve decided that building a cleanroom is required for the development of your products the first question that must be answered is which class of clean room is needed.
The current standard US and Canada is the ISO classification system ISO 14644-1. The lower the ISO class the requirements for cleanliness go up.
Second, once the classification is identified this will determine your layout and design. For example, ISO 8 does not require you to enter through an airlock, while ISO 6 does.