Under assessment as a single person, each spouse is treated as a single person for tax reasons. With this option:
Under the separate assessment option, the tax affairs of spouses are independent of each other. The difference between separate assessment and assessment as a single person is that under this option, some tax credits are divided equally between you. These tax credits are:
appropriate spouse. Any tax credits other than the PAYE tax credit and employment expenses that are unused by one partner can be claimed by the other spouse. The tax credits under this method are not usually adjusted until after the end of the tax year.
The joint assessment (or "aggregation") option is usually the most favorable basis of assessment for a married couple. This option is automatically given by the tax office when you advise them of your marriage but this does not prevent you from choosing to be assessed individually or separately. Under this option, the tax credits and standard rate cut-off point can be allocated between spouses to suit their own circumstances.
If only one spouse has taxable income, all tax credits and the standard rate cut-off point will be given to the spouse with the income.
If both of you have taxable income, you can decide which of you is to be the assessable spouse. You then ask the tax office to allocate the tax credits and standard rate cut-off point between you in whatever way you wish.
If your tax office does not get a request from you to allocate your tax credits in any particular way; the tax office will normally give all the tax credits (other than the other partner's PAYE and expense tax credits) to the spouse being assessed. The spouse being assessed must complete the return of income for the couple and is chargeable to tax on the joint income of the couple.
If one spouse is self-employed, joint assessment can still apply. The flexibility this option brings can be very convenient - especially if one of you pays tax under the PAYE system and the other pays tax under the self-assessment system. Under joint assessment, you let your circumstances determine if most of the tax should be paid under PAYE or in a lump sum on assessment. This is determined by the way in which the tax credits are allocated. If you choose to pay most of your tax under PAYE, the tax credits (apart from the PAYE tax credit and employment expenses), should be offset against the self-assessment income.
The choice about who becomes the assessable spouse is made by both of you. All you need to do is to inform Revenue which of you is to be the assessable spouse. If you have not made your nomination, the assessable spouse with the higher income in the latest year for which details of both spouses' incomes are known becomes the assessable spouse. This person continues to be the assessable spouse until both of you jointly elect that the other spouse is to be the assessable spouse or until either of you opts for either separate assessment or assessment as a single person.
In my opinion it is much more beneficial to be assessed jointly rather than individually so I would suggest that if you are married but taxed individually you should notify the Revenue if you have not done so already and ask them to change your method of assessment to joint. And who knows if you forward them your P21’s for the last few years for both of you (the year of marriage will not be factored into calculations for joint assessment) you may even be due some tax back.