Self Assessment season capacity planning for practices

Self Assessment capacity planning means counting the returns you must file by 31 January 2027, estimating the hours each one needs, and checking that total against the staff time you actually have. Do it in early November, while there is still time to set client cut-offs, chase records and bring in help. This guide gives a simple model, HMRC’s latest filing data and the dates that shape the season.

How big is the 2027 Self Assessment peak?

HMRC expects about 12 million people to file a 2025 to 2026 return by 31 January 2027, according to its 100 days to go release of 23 October 2026. Last season’s figures show how late the work arrives.

The figures come from HMRC’s press release of 3 February 2026. Your own practice will follow a similar curve unless you change client behaviour.

Build a simple capacity model

A capacity model is a short spreadsheet with three columns: return type, number of clients and hours per return. Use your own time records for the hours. The numbers below are an example only.

In this example the total is 180 + 240 + 150 + 150, which is 720 hours. Add 15% for review and queries, and you need about 830 hours.

Then work out the hours you have. Count each preparer’s productive hours per week, not their contracted hours, from now to 31 January. Take off Christmas closure, holidays and time for other deadlines such as December VAT and January Corporation Tax. If the hours you need exceed the hours you have, decide now how to close the gap.

Set client cut-offs that protect January

Client cut-offs move work out of the final fortnight. A practical pattern is:

1. 30 November: records deadline for all clients. Say in writing that later records may carry a rush fee.
2. 18 December: target date for returns that may pay through the tax code.
3. 30 December: HMRC’s online deadline for clients with PAYE income who owe under £3,000 and want it collected through their tax code.
4. 15 January: last date to accept records with a guarantee of filing.
5. 31 January: HMRC’s filing and payment deadline.

Put these dates in your engagement letters and in an automated reminder sequence. See the December 2026 deadlines for the other dates running alongside.

Fix authorisations and access early

New clients need agent authorisation before you can see their records. With Online Agent Authorisation, HMRC posts a code to the client within 7 working days. The client must give you the code within 30 days of the letter, or it expires. The paper alternative is form 64-8, set out in HMRC’s agent authorisation guidance, last updated 7 October 2026.

Run an authorisation check across your client list this month. Anything outstanding in January will cost time you do not have.

Plan for the extra work this season

Three changes add work to 2025 to 2026 returns:

• Making Tax Digital for Income Tax. Clients already in MTD still file their return as usual by 31 January 2027, HMRC says. Their 2025 to 2026 qualifying income also decides who joins from April 2027 under the £30,000 threshold.
• Winter Fuel Payment charge. Pensioner clients with total income over £35,000 repay the payment through their bill. Check it appears on each return.
• New clients from closures and mergers. Price and schedule late joiners separately, not on top of existing work.

Ways to close a capacity gap

If the model shows a shortfall, pick from these options in this order:

• Move work earlier. Contact clients whose records arrived late last year first.
• Cut low-value work. Review clients who are not profitable. See how to sack a client.
• Outsource. Use a vetted outsourcing partner for routine preparation and keep review in house.
• Hire temporary staff. Book them now. Agency availability shrinks in December.
• Charge for late records. A clear rush fee changes behaviour.

What firm owners should do now

1. Pull a list of every 2025 to 2026 return you will file.
2. Group by complexity and apply your own hours per type.
3. Compare hours needed with hours available to 31 January.
4. Email clients the 30 November records deadline this week.
5. Check agent authorisations and missing UTRs.
6. Agree who covers queries in the last week of January.

Remind clients what being late costs: £100 at once, then £10 a day after three months up to £900, then 5% or £300 at six and twelve months, according to GOV.UK.

Frequently asked questions

When should a practice start Self Assessment capacity planning?

Early November is the latest sensible point. It leaves time to set a records deadline, chase authorisations and book help before December.

How many Self Assessment returns does HMRC expect for 2025 to 2026?

About 12 million, according to HMRC’s release of 23 October 2026. The deadline is 31 January 2027.

How many people filed on deadline day in 2026?

HMRC received 475,722 returns on 31 January 2026. The busiest hour was 5pm to 6pm, with 32,982 returns.

How long does agent authorisation take for Self Assessment?

Using Online Agent Authorisation, HMRC posts the code to the client within 7 working days. The code must be used within 30 days of the letter.

What is the penalty if a client misses 31 January?

An initial £100 penalty, then £10 a day after three months up to £900, then further penalties at six and twelve months.