If you are applying for immigration permission (visa) for more than six months you must pay an additional fee to entitle you to free health care under the National Health Service. You will need to pay £776 for each year of leave you apply for and and extra £338 for any period less then 6 months. Any dependants applying with you will need to pay the same amount.
The calculation for IHS is based on the length of the visa you will be granted and not just the length of your course or the additional period you might be adding on to an existing visa.
Use the calculator on the UKVI website to check how much you will need to pay and you can confirm the current charges here.
You pay the charge as part of the Student visa application process and the form will calculate how much you need to pay based on the information in your CAS.
You are required by to pay the charge even if you have private medical insurance.
If you are applying to add a placement or move to an integrated masters course then you will likely be charged for some of the same period of time you have already paid for. Unfortunately, there is no way around this as the payment of the IHS is mandatory and this is just how the process works. However, you should be eligible for a refund of the overlapping period of IHS coverage provided the overlap in coverage between your two applications is 6 months or more. This refund should be processed automatically within 6 weeks of you obtaining a decision on your new application so you should get your money back eventually.
Incorrect IHS calculation
If the form calculates the incorrect amount you may receive an email from UKVI asking you to pay a top-up fee and if you don't pay the top-up fee your Student visa application could be refused so you should check all your email accounts regularly.
For further information about the immigration health charge visit the UKVI website