IR35 Investigation - is duration HMRC can investigate absolute or by contract length?

[Pring]

I understand that for IR35 investigations HMRC can extend an enquiry for 4 years for innocent mistakes, 6 years if a contractor has been noticeably careless or 20 years if they think there's been fraudulent activity or deliberate tax avoidance. Assuming a 6 year investigation, what's the impact if a past contract that they find to be Inside IR35 spanned the boundary of those 6 years. Is the whole contract duration at risk, or just the period within six years?

A contrived example, where client A is retrospectively found to be Inside IR35. Is the potential liability for Current Year -4 and -5, or -4,-5,-6,-7.

Current Year : Client C - Year 2. Outside IR35.
Current Year - 1 : Client C - Year 1. Outside IR35.
Current Year - 2 : Client B - Year 1. Outside IR35.
Current Year - 3 : Client B - Year 2. Outside IR35.
Current Year - 4 : Client A - Year 4. Considered outside at time but determined to be Inside IR35 by investigation.
Current Year - 5 : Client A - Year 3. Considered outside at time but determined to be Inside IR35 by investigation.
--- 6 Year Boundary ----
Current Year - 6 : Client A - Year 2. Considered outside at time but determined to be Inside IR35 by investigation.
Current Year - 7 : Client A - Year 1. Considered outside at time but determined to be Inside IR35 by investigation.

[BR14]

the whole 20 years.

[Pring]

the whole 20 years.

Assume contracts were IR35 reviewed by QDOS, so unlikely to be considered fraudulent.

Though on that note, I'd thought that, assuming company is kept open but only works inside IR35 from start of next year, it'd 'only' have to keep IR35 insurance for 6 years from the last outside IR35 contract to be safe from retrospective actions. Keeping for 20 would be painful...

[northernladuk]

IR35 is considered on a contract by contract basis so, the question in the title, the answer is the latter.

If you ever quit contracting the rule of thumb seems to be to keep your insurance running for 1 full tax year after you've closed the company just in case. There is no evidence to date of HMRC chasing a closed company but for £250 it's daft not to pay it and not be the first one that gets hit.

Nice simple answer for you