Data Sources & Update Policy
Casvia aims to reflect published UK rules as accurately as possible. This page explains where our data comes from, how often we update, and why tax years matter.
Our sources
We prioritise official UK government and public bodies. We do not use random blogs or unverified third-party sites as primary data sources.
HMRC
Her Majesty's Revenue & Customs — Income Tax, National Insurance, Capital Gains Tax, Dividend Tax, Inheritance Tax, ISA rules and pension tax relief.
GOV.UK
The official UK government website — tax rates, allowances, thresholds, Stamp Duty (SDLT), Student Loan repayment thresholds and Child Benefit rules.
ONS
Office for National Statistics — CPI inflation data, retail prices, earnings statistics and economic indicators used in our inflation calculator.
Bank of England
The UK's central bank — official Bank Rate, mortgage market data and inflation targets referenced in our mortgage and savings calculators.
Scottish Government
Devolved Scottish tax rules — Land and Buildings Transaction Tax (LBTT) and Scottish Income Tax bands where applicable.
Welsh Government
Devolved Welsh tax rules — Land Transaction Tax (LTT) for property purchases in Wales.
We also reference MoneyHelper (moneyhelper.org.uk) and the Financial Conduct Authority (FCA) (fca.org.uk) for consumer guidance and mortgage regulation context, but these are not primary sources of tax rates or thresholds.
How often do we update?
UK financial rules change throughout the year. We review and update our calculators when any of the following occur:
- Annual tax-year review — every April, when the new UK tax year begins (6 April). We verify all tax bands, allowances and thresholds against HMRC and GOV.UK.
- UK Budget — when the Chancellor announces changes to tax rates, allowances or thresholds (usually March/November).
- Autumn Statement / fiscal updates — additional fiscal events that may change tax rules.
- Official threshold changes — when HMRC, GOV.UK or devolved governments publish updated rates mid-year.
- Major regulatory changes — FCA rule changes, Bank of England policy shifts, or significant mortgage market reforms.
Each tax-sensitive calculator page displays the tax year it currently reflects (e.g. "2026/27"). If a rule changes mid-year, we update the affected calculators and note the change.
Why tax years matter
UK tax thresholds, allowances and rates are set by tax year, not calendar year. The UK tax year runs from 6 April to 5 April the following year. For example, the 2026/27 tax year runs from 6 April 2026 to 5 April 2027.
This matters because:
- Income Tax bands, Personal Allowance and National Insurance thresholds can change each tax year.
- ISA allowances and pension annual allowance are set per tax year.
- Stamp Duty rates and reliefs can change at Budget events, not necessarily at the tax year boundary.
- Student Loan repayment thresholds are reviewed annually.
Using last year's thresholds can give you a significantly wrong result. Always check that the calculator reflects the tax year you're interested in.
Our approach
Casvia aims to reflect published UK rules as accurately as possible, but our calculators are educational tools. They are designed to help you understand how UK financial rules work and estimate outcomes based on the inputs you provide.
Important limitations:
- Calculators use standardised assumptions and may not capture every individual circumstance (e.g. complex tax codes, multiple employments, unusual income types).
- Interest rates and investment returns are assumptions you enter — they are not predictions or guarantees.
- Mortgage affordability estimates are illustrative — actual lending decisions depend on lender criteria, full expenditure assessment and credit history.
- Rules can change between updates. Always verify against the latest official guidance before making financial decisions.
We do not provide personal financial, tax or investment advice. If you need advice tailored to your situation, speak to a qualified financial adviser, tax professional or mortgage adviser.
Spot an error?
We take accuracy seriously. If you spot a discrepancy between our calculator and an official source, please let us know. We review all correction reports and update promptly when verified.
Last reviewed: September 2026 · Current tax year: 2026/27 · Primary sources: HMRC, GOV.UK, ONS, Bank of England, Scottish Government, Welsh Government