Nothing happens to your site: it keeps running as normal, and the extra bandwidth is charged at £0.05 per GB plus VAT. Plot never throttles or caps a site.
Every site includes 100 GB of bandwidth a month, on both plans. As a rough guide that covers around 85,000 visits, depending on page size and how much media people load.
How the extra is billed:
It is added to your next monthly invoice after the usage period ends.
For example, if a big on-sale month takes you to 300 GB, that is 200 GB over, so £10 plus VAT for that month.
We email you as you get close to 100 GB.
If there is a charge, you get at least three full days after the charge notice before payment is collected.
Other things people ask:
Will my site slow down or go offline during a lineup announcement or on-sale? No. Hosting is built for announcement and on-sale traffic, and going over 100 GB does not change how your site runs.
Does one busy month change what I pay after that? No. Bandwidth resets each month, so a busy on-sale month may cost a little more and a quiet month costs nothing extra.
Is the allowance shared across my sites? It is 100 GB per site.
Is it different on Managed? No. The allowance and the rate are the same on Core and Managed.
Hosting itself is covered in is hosting included, and plan prices are in how much is Plot.