FOI reference: FOI-2026-3791
You asked
Under the Freedom of Information Act 2000, I request the following information used in compiling the mortgage interest payments (item 410201, MM23 series DOBQ) and depreciation (item 410801, MM23 series CHOO) components of the Retail Prices Index. I would be grateful for it in machine-readable form (CSV or XLSX).
1. Arithmetic house-price series used in the RPI
The "Calculating the Retail Prices Index" guidance (section 5, Formula 4) describes an arithmetic house-price input derived from the UK House Price Index, with Nationwide movements used for the most recent month. Please provide:
a) the monthly arithmetic UK house-price series supplied for the RPI, as each value was used in compiling each published index (the operational vintage, not the series as later revised), for every month from February 2003 to the latest published index;
b) the Nationwide series used, with its seasonal-adjustment status and the index values used, for February 2017 to date, and the Halifax series used before February 2017, back to February 2003;
c) the house-price values (H_t, the Formula 4 output) used in each monthly MIP and depreciation calculation from January 2015 to date;
d) the January 2026 anchor and the bridge used when the improved UK HPI imputation method was adopted for RPI housing inputs from the February 2026 index (released 25 March 2026). This means the values on the old and the new basis for the overlap months, and any splicing factors.
2. Mortgage-age and repayment-type weights
Please provide, for each RPI index year from 2003 to 2026:
a) the mortgage-age (duration since purchase) weight vector used for the 276 monthly purchase vintages, or the duration-of-residence distribution from which it is derived;
b) the repayment-type split (repayment versus interest-only/endowment), by mortgage age if it varies;
c) the interest rate or rates used to construct the outstanding-balance schedule of repayment mortgages, and the formula convention (for example annuity with monthly compounding);
d) any household-coverage scaling factors applied, if they vary within a year.
3. Average effective rate (AER) and its forecast specification
The guidance states that the Bank of England supplies a forecast of the average effective rate, using quoted two- and five-year fixed rates with 24- and 60-month averaging and variable-rate information, because the current month's effective rate is not available in time. Please provide, for January 2015 to the latest published index:
a) the specification of that forecast:
- the quoted-rate series used (Bank of England codes, including loan-to-value bands);
- whether the averaging windows include the current month;
- the variable-rate series used;
- the coefficients, weights and intercepts, with the dates on which they changed;
b) the AER value used in each monthly MIP calculation as it was at the time of compilation, including both January values (on the old and new weights) for each year, if these differ from the values published in ad hoc release 2643 (19 February 2025) and FOI-2026-3390 (26 February 2026);
c) the January-fixed mortgage-stock weights used to combine lender or product effective rates into the AER, for each year from 2015.
If any part of this request would exceed the cost limit, please prioritise the items in this order: 1(a), 3(a), 2(a)–(c), 3(b), 1(d), then the remainder. Please also advise how the request could be narrowed. If any information is held by the Bank of England rather than ONS, please say so, so that I can direct that part to the Bank.
We said
Thank you for your request.
Unfortunately, it is not possible for us to action your request within the FOIA cost limit timeframe of 24 working hours. Section 12 of the Freedom of Information Act 2000 (FOIA) applies in this request as we would need to search back through years of correspondence and archived material to manually retrieve and determine which exact datasets and components were used.
You may wish to narrow your request so we may answer within the cost limit. We could reasonably provide parts 1(a) and 1(b) of your request dating back to 2017.
The data for 1(a) dating back to 2017 are available on the ONS website: House Price Index, unrevised arithmetic mean for Great Britain: January 2017 to August 2026.
The data for 1(b) dating back to 2017 are accessible on the Nationwide website. The Nationwide index values used are those in the "INDEX Q1 1993=100" column in the "UK Monthly Index" file published by Nationwide. These data are available for the time periods requested and publicly available to download from Nationwide's website: Nationwide HPI News - Data & Resources. The seasonal adjustment status for this data is: Non-seasonally adjusted.
In a refined request we could also provide an update to the information requested in 3(b) of your request within the cost limit.
Please note that the information requested in the remainder of part 3 of your request is derived from data provided to us by the Bank of England with the restriction that it would not be shared further. We therefore would not be able to share this data in a refined request. Similarly, for 2(a) and 2(b), the information requested is derived from data provided to us by UK Finance under similar restrictions we are not permitted to release this information within the contractual terms.