Generally speaking, anyone is entitled to contribute to a registered pension scheme. The only issue is whether or not they receive tax relief on the contributions.

UK residents - can contribute up to the higher of £3,600 or 100% of their relevant UK taxable earnings. However, where the total contributions paid (by your client and/or their employer) in any tax year exceed the annual allowance, there will normally be a tax charge of 50% on the excess over the annual allowance.

Personal contributions made in excess of 100% of relevant UK taxable earnings do not receive tax relief and do not count towards their annual allowance.

Non-UK resident individuals - entitled to contribute but will not be entitled to any tax relief on contributions. Non-resident but have been resident or had earnings chargeable to UK income tax in the last five years - entitled to contribute but will only receive tax relief on contributions up to £3,600 gross (£2,880 net).

Employer contributions
There is no limit on how much your client’s employer can contribute to your client’s SIPP. However, where the aggregate of all employer and personal contributions in the tax year is over the annual allowance, there will generally be a tax charge to your client personally of 50% on the excess.

The exception will be for any contributions made in the tax year in which benefits are drawn in full from the SIPP, in which instance there will be no personal tax charge irrespective of the size of employer contribution.

For employer contributions to receive tax relief, they must be 'wholly and exclusively for the purposes of trade' as defined by ICTA 1988. HMRC have issued guidance for employers to help define this, including pages on the online RPSM.

Notes
Contributions can not be refunded without being an unauthorised payment unless the contribution was made in error or are in excess of your client's relevant UK taxable earnings.