SIPPs and adjusted income

Hi there, Where income is above £100k am I correct in thinking that SIPP voluntary personal contributions reduce the adjusted net income as far as it applies to determining eligibility for Early Years free childcare and Tax-Free Childcare Benefit? As a concrete example if someone who already files Self Assessment expects to earn £109k by the end of the financial year, and £2k of that is an existing Nest pension contribution (believe this is relief at source), and then one makes a £12k deposit into a SIPP, my understanding is: 1. The adjusted net income is considered to be £95k (109-2-12) and thus eligibility for Early Years childcare schemes is not affected 2. The SIPP provider makes a basic relief claim automatically so 20% of £12k automatically added to account 3. Come Self-Assessment time, the additional £12k contributed is listed and the remaining higher rate relief above basic relief applied So apart from the timing differences there should be no net difference compared to increased net pay pension contributions? However I am also unsure about this guidance https://www.gov.uk/tax-on-your-private-pension/pension-tax-relief which seems to suggest if the amount is above £10k that Self-Assessment filing won't be sufficient but one would have to write to HMRC separately to notify about the amount? If this is correct, why is self-assessment filing not enough and can this be done after the payment? There's no guidance on timing I see on that page. Many thanks

Hi G5000,
If you make payments into an approved pension scheme from your net then yes this will reduce your adjusted net income.
Yes the pension provider claims the 20% relief at source.
Yes the relief will be given when you submit your Self Assessment tax return.
If you complete a Self Assessment you do not need to submit evidence on the pension payments with the tax return.
If additional information is needed we will write to you. 
Thank you.