Hi @Jasmine,
Your understanding is correct. The 12-week register method becomes invalid if the number of car parking benefits increases by more than 10% compared to the previous FBT year. The key measure is the number of car parking spaces (or the number of employees allowed to park if this is less), not the total number of employees in your organisation.
In your example, if you have 30 car parking spaces available and this number remains unchanged, the register stays valid even if your total workforce grows beyond 10%. The number of car parking benefits is determined by the available parking spaces, not by how many employees you employ overall. As long as the number of spaces (or employees permitted to park, whichever is lower) doesn't increase by more than 10%, the 12-week register remains valid for that FBT year.
It's worth noting that even if the 10% threshold isn't exceeded, you'll need to complete a new 12-week register every five years as part of the regular requirements for this method.
You can find detailed information about the 12-week register method and other valuation methods for car parking fringe benefits on our car parking and FBT page.