New this month New house price data for May 2015 are published this month. The monthly and quarterly reference table (3.6 Mb Excel sheet) has been updated to include data for May 2015. Revisions this month There are small revisions to the seasonally adjusted series for the last 12 months, which are expected from the monthly seasonal adjustment process. Revisions next month Small revisions are expected next month for the April 2015 and May 2015 HPI figures as scheduled. These reflect quarterly submissions delivered by a small proportion of mortgage lenders.

Relevance of the ONS House Price Index The ONS HPI is an important measure of house price inflation for the UK and together with the Land Registry HPI, it is one of the main house price indices used by central and local government to support decision making in the UK. Other users include private individuals, surveyors and analysts in financial institutions. The ONS HPI is also an important input into the housing cost component of RPIJ and RPI retail price indices. Each month a customised HPI delivery is produced using a sub-sample of the full data set for use in RPIJ and RPI.

Revisions policy At the end of every quarter, as well as releasing final figures for the latest month, we revise the figures from the previous 2 months. This is done because some mortgage lenders, which account for 1 to 2% of all records, provide their data on a quarterly rather than monthly basis. Additionally, data will be revised for the previous month if more than 1,000 additional cases are received in a subsequent month. In July 2013, the methodology used to seasonally adjust the HPI was updated following a review and brings the HPI in line with our best practice for seasonal adjustment. Seasonal factors are now estimated on a monthly basis and therefore may result in small revisions to the previous 12 months data. This updated process improves the accuracy of the seasonally adjusted figures. Other revisions to historical data (other than those currently due for revision) will be made only if the revision is substantial. In all cases, the revised figures are labelled with a 'R' and the reason for the revision explained under the 'New this month' section of the background notes.

Methodology Data sources Since October 2005 the ONS HPI (formerly the DCLG HPI) has been based on a sample of mortgage completions data from the Regulated Mortgage Survey (RMS) as collected by the Council of Mortgage Lenders (CML). The number of transactions received from the RMS is affected by the total number of mortgages completed for house purchase in any period. During 2011 the sample covered 65-70% of all UK mortgage completions. Quality A Quality and Methodology Information (QMI) (131.8 Kb Pdf) paper for the HPI describes in detail the intended uses of the statistics presented in this publication, their general quality and the methods used to produce them. Price methodology The ONS HPI is mix-adjusted to allow for differences between houses sold (for example type, number of rooms, location) in different months within a year. House prices are modelled using a combination of characteristics to produce a model containing around 100,000 cells (one such cell could be first-time buyer, old dwelling, one bedroom flat purchased in London). Each month estimated prices for all cells are produced by the model and then combined with their appropriate weight to produce mix-adjusted average prices. The index values are based on growth rates in the mix-adjusted average house prices and are annually chain linked. More information on the model used is available via the hedonic model methodology paper (246.4 Kb Pdf) published on the HPI user guidance webpage. Re-weighting The ONS HPI is a weighted Laspeyres-type index. In January of each year the index weights are updated based on the relative numbers of transactions during the previous three years, which are grossed to total transactions obtained from Land Registry. Applying new weights ensures that the index keeps up to date with changes in the types of properties that are being purchased, and therefore reflects the price of the average property. A high level summary of the weights used in the calculation of the ONS HPI can be found in the HPI weights summary reference table (74 Kb Excel sheet) . One consequence of changing the weights every year is that the mix-adjusted house prices cannot be compared between years as the weights are different. The index itself is constructed on a chain-linked basis, which enables year-on-year comparisons to be made. This means that the year-on-year change in the index for June 2011, say, is effectively the change in the average price from June to January 2011 (using the weights for 2010) combined with the change in the average price from January to June 2011 using the weights for 2011. Therefore, the year-on-year change in the index is not the same as the year-on-year change in the mix-adjusted average price. More information on the HPI methodology is available on the GOV.UK website. Seasonal adjustment The housing market shows seasonal effects that affect house prices. For example, prices have tended to be higher during the summer months than during the winter months. These seasonal effects are estimated and adjusted for in order to calculate month-on-month price changes. Seasonally adjusted figures are provided at a national level in Table 7 alongside the non-seasonally adjusted figures of the other tables. Seasonal adjustment is performed each month and reviewed each year, using the standard and widely used software X-13-ARIMA. Seasonally adjusted house price estimates are used to report monthly percentage changes. All other figures such as annual rates of change and average house prices are based on non-seasonally adjusted estimates, unless otherwise stated.

Other house price statistics Currently there are a number of different sources of house price statistics published in addition to the ONS HPI. There will be differences in the data published by each source as there are differences in both the data and methodology used. Therefore the ONS HPI is not directly comparable with these other indicators. Further details on the differences between official house price statistics can be found in the article Official House Price Statistics Explained (974.4 Kb Pdf) . Land Registry house price index All residential property transactions in England and Wales are recorded by Land Registry. These transactions are used for calculating the Land Registry index. This index is based on repeat-sales regression, which calculates the change in price of any property transacted twice since 1995. Therefore new build properties are excluded from the index. Land Registry publishes indices at a sub-regional level. The Land Registry HPI is normally published on the 20th working day of every month, and refers to all transactions of the preceding month. The Land Registry HPI can be accessed via the Land Registry's website. Registers of Scotland official quarterly housing market statistics Registers of Scotland records all the property transactions in Scotland. It produces average house prices based on arithmetic means of these transactions, which is published as the quarterly housing market statistics in the second month after the month to which the figures refer to. Northern Ireland residential property price index The Land and Property Services assisted by the Northern Ireland Statistics and Research Agency (NISRA) publish a quarterly residential property prices index (RPPI) for Northern Ireland. The index measures change in the price of residential property sales recorded by Her Majesty's Revenue and Customs. This is a new official statistic, first published in quarter 1 of 2012. Halifax house price index and Nationwide house price index Both Halifax and Nationwide produce house price indices based on their own mortgage approvals only and therefore, like the ONS HPI, will not include any cash transactions. They both have UK-wide coverage, and since the Halifax and Nationwide use only their own in-house data they can process them immediately and do not have to await the receipt of data from other lenders. This means that they are more timely than the ONS HPI. LSL Acadata house price index The LSL Acadata (previously the LSL Property Services/Acadametrics) HPI is the only house price index to reflect all transactions, as opposed to data samples, and provides mix and seasonally adjusted results at national, regional and county or unitary district or London borough levels. The index can be accessed at Acadata.

Accessibility This bulletin includes the May 2015 data. Future publication dates for this statistical bulletin are available via the release calendar.

General Details of the policy governing the release of new data are available from the Media Relations Office. Also available is a list of the names of those given pre-release access to the contents of this release. Follow us on Twitter or join us at Facebook