Since HMRC closed CATO (its own free filing service) on 31 March 2026, every UK company has to file its CT600 through third-party software. Prices range from free to several hundred pounds a year, and the differences between products matter more than most directors expect. Here is what to actually check before you commit.
1. Is it on HMRC's recognised list?
HMRC maintains a public list of commercial software it has tested against its Corporation Tax filing gateway. A product not on that list cannot file with HMRC at all, whatever else it promises. This is the first thing to check, not the last.
2. Does it check your figures before it lets you file?
The single biggest difference between products is not price, it is how much they validate before submission. Good software will flag, before anything is sent to HMRC:
- A balance sheet that does not balance
- Losses claimed in excess of what is actually available to carry forward
- Dates that do not make sense (a period ending before it starts, gaps between periods)
- Missing contact details HMRC's gateway will otherwise reject on submission
- A period already filed, being resubmitted without being flagged as an amendment
Software that skips straight to "generate and submit" with no checks in between will happily let you file something HMRC rejects, or worse, something HMRC accepts but that is quietly wrong. A rejection at least tells you something is off; a wrong figure that HMRC accepts can sit unnoticed until an enquiry years later.
3. Does it handle marginal relief automatically, and correctly?
Since April 2023, UK corporation tax has three bands, not one: 19% up to £50,000 of profit, 25% above £250,000, and a sliding-scale "marginal relief" in between, using HMRC's standard fraction (currently 3/200). These thresholds are also divided between associated companies and pro-rated for periods shorter than 12 months, two details that are easy to get wrong by hand and easy for cheap software to get wrong silently. Ask, specifically, whether marginal relief and the associated-companies adjustment are both handled, not just the headline rates.
4. Does it actually support the capital allowances you need to claim?
Most software calculates Annual Investment Allowance and other capital allowances correctly as a number. Whether it can actually FILE that number is a separate question, because of how HMRC's tax computation format ("iXBRL") requires certain figures to be tagged. This has genuinely tripped up more than one small filing tool, including an earlier version of our own CT600 Filer, which currently refuses to file a non-zero AIA claim rather than risk submitting something HMRC's gateway will reject. If your company claims capital allowances beyond the basics, ask the software provider directly whether that specific claim type is supported for filing, not just for calculation, and test it in HMRC's test service before you rely on it for a live return.
5. Is the pricing model actually transparent?
Watch for three common patterns:
- The 12-month subscription trap. Several products that look like a modest monthly fee actually require a 12-month commitment, so cancelling after one filing does not stop the billing.
- Per-filing add-ons. A headline price for the CT600 form itself, with the accounts (iXBRL) filing charged separately, can roughly double the real cost.
- "Was free" products going paid. Several previously free CT600 tools have introduced subscriptions since HMRC closed CATO, understandably, since HMRC's own free option disappearing increased demand across the board. Check the CURRENT price on the provider's own site, not what you remember or what a review site says, since this whole market has been moving fast.
6. What actually happens if HMRC rejects your return?
Every product will occasionally hit a rejection, whether from a genuine error in the figures or a technical formatting issue. The difference is what you see when it happens: some products show you HMRC's raw technical response (an XML error referencing an internal tag name most directors have never heard of), others translate it into plain English and point you at the actual field to fix. Ask to see an example, or try the test service deliberately with a figure you know is wrong, before you're relying on the software under time pressure near a filing deadline.
7. Is it actually built for a company like yours?
A single small trading company with straightforward, single-stream income has very different needs from a multi-client accountancy practice, or a company with a group structure, chargeable gains, or an active HMRC enquiry. Software priced and designed for practices managing dozens of clients is rarely the most efficient choice for one company filing its own return once a year, and the reverse is equally true. Match the tool to the job, not just to the badge on the price tag.
Frequently asked questions
Does CT600 software need to be HMRC-recognised?
Yes. Since HMRC closed its own free filing service in March 2026, every company must file through software HMRC has tested and recognised. Check the provider appears on HMRC's current list.
Is the cheapest CT600 software the right choice?
Not necessarily. Whether the software checks your figures before submission and explains rejections in plain English matters more than the price tag alone.
Can any CT600 software handle capital allowances?
Most calculate them correctly. Whether a product can actually file certain claims, particularly Annual Investment Allowance, depends on how it has solved HMRC's tagging rules, and varies between products. Ask directly, or test first.