Your starting point should always be the Code of Practice, which is here: https://www.gov.uk/guidance/immigration-rules/immigration-rules-appendix-j-codes-of-practice-for-skilled-work
Each occupation code says how much you should be offered. Beware of the catch – those salaries are based on the certain number of working hours. If your hours differ, your salary may need to be amended accordingly (but usually still subject to some minimum).
You will see New Entrant level and Experienced level. The Code of Practice explains when to use them. For example, Tier 4 migrants switching to Tier 2 can be offered New Entrant one (lower, but for the same reason more attractive to sponsors). Those who already have a Tier 2 General visa and applying to extend (or change to another Sponsor) fall under the Experienced level.
If you are eligible to apply as a New Entrant, your minimum salary must be as on your code but minimum £20800 – whichever is higher.
If you have to apply as an Experienced worker, the minimum as on the Code or £30000, again whichever is higher.
If you are applying for Indefinite Leave, the minimum is still as on the Code but also minimum £35000 – whichever is higher. From 6 April 2018, it is £35500 and then going up again every year.
If your salary is minimum £35000 (£35500 from April 2018), but your Code says it must be, say, £40000, then you must be earning £40000, even if your last COS was issued on the basis of £35000.
If you are a student and considering what’s next after Tier 4 visa, we have a webpage here: http://www.1st4immigration.com/whats-next-after-tier-4.php or we have the general homepage with the links to all subjects and blog posts: www.1st4immigration.com
For individual advice or to make an application please contact us: firstname.lastname@example.org. We respond emails on the same working day and have almost 100% success rate!
1st 4Immigration Ltd, authorised by the OISC, ref 200800152. We operate at the highest Level 3 of expertise. Office address: 68 King William Street, City of London, London, EC4M 7DZ.