There are two routes of eligibility for ESA. One is 'contributary' and relies on the claimant's NI contribution history and one is 'income related' for people who do not meet the contributary requirements.
In addition, it is possible to get contributary ESa with an income-related top-up for claimants who meet the contributary requirements but are on low income. The income, assets and working hours of any partner who lives with the claimant will also be taken into account.
24 hour work rule for ESA(IR) Asset rule for ESA(IR) If people are on income-related ESA, either alone or as a top-up, they might be entitled to a little more money and are also entitled to other things - such as free NHS prescriptions.
A further point to note is that contributary ESA is limited to 365 days for claimants in the WRAG but no such limit applies to people on income-related ESA.