United Kingdom - House Price Index for New Homes

For the U.K., the Nationwide House Price Index is an index created by Nationwide Building Society with the purpose of tracking price movements in the residential housing market, using its own mortgage data.

The data used are for mortgages that are in the post-survey, approval stage. In contrast to using prices calculated at the time of mortgage completions, these prices should give a more timely indication of price trends in the housing market. The data also excludes house prices over £1 million. The house prices are mix adjusted - i.e., a representative house price over time rather than the simple average price is tracked over time.

Monthly time series are available to measure the mix-adjusted average house price for all houses in the U.K. Nationwide also publishes quarterly series which provide a detailed breakdown based on the category of buyer (first-time buyer and former owned properties) and mortgage burden (repayment as percentage of income).

National

• Measurements:
  • British pounds (GBP)
  • Fixed-base index relative to 1993Q1 (Index 1993Q1=100)
• Adjustments:
  • Seasonally adjusted (SA)
  • Not seasonally adjusted (NSA)
• Native frequency: Monthly
• Start date: Uniformly 1991m1
• Geo coverage: Country

The source writes:

Source

All house price information is derived using Nationwide mortgage data. This data is extracted monthly for mortgages that are at the approvals stage and after the corresponding building survey has been completed. Approvals data is used as opposed to mortgage completions since it should give an earlier indication of current trends in prices in the housing market.

Cleaning

Nationwide house price series utilize only residential property information. In addition, properties that are not typical and may distort the series are also removed from the data set. Therefore, the following criteria is used to select which properties to include:

• House purchases - remortgages and further advances are excluded
• Properties sold at true market prices - right to buy sales at discounted price are excluded
• Floor size has to be within specified limits for a give type of property - e.g. a detached house has to have at least 400 sq. ft. floor area

Sample Size

The number of cases that are used to calculate the average price for a given month will depend on the volume of monthly mortgage activity and out of these the cases that meet the criteria in the cleaning process. The monthly sample size will therefore vary from month to month. Nationwide has sufficient sample size to produce a representative house price series. N.B. Net lending figures quoted at our half yearly and annual results are not a guide to our sample size. Sample size is based on the number of new loans we write i.e. the amount of gross lending for house purchase(remortgage cases are excluded).

The Nationwide Building Society is the 4th largest mortgage lender in the UK by stock. This allows us to be confident that the series based on Nationwide mortgage data is representative of the whole house market.

The quarterly UK series for all houses uses 3 months of data and hence a much larger sample than at the month. The samples sizes for the other quarterly series will depend on what it is they are measuring, for example the series for first time buyers only considers properties being brought by first time buyers and hence this will have a smaller sample size than that used for the whole of the UK. It is for this reason that detailed breakdown of house prices are produced quarterly.

Calculation

The price of a property will depend on the characteristics of the property. These characteristics could include physical properties of the house, like its design, but other aspects such as the type of neighborhood the house is located in will also contribute to the price someone is willing to pay. Using mortgage data, the Nationwide house price system can relate all the observed combinations of these factors and relate them to the price of which the house was sold for. From this, the model can estimate how much on average a house would cost given a set values for these characteristics, in particular a set of characteristics that describes the 'typical' house. This typical house does not physically exist, it is an 'average' house across all the characteristics that the model uses. This method is repeated on data sets at different points in time and changes in the price of this typical house reflect only the price changes over the same time periods, and not the mixture of properties sold in the current or previous periods.

Seasonal Adjustment

House prices are slightly seasonal - that is, prices are higher at certain times of year irrespective of the overall trend. This tends to be in spring and summer, when more buyers are in the market and hence sellers do not need to discount prices so heavily, in order to achieve a sale. The effect on prices over the year is of the order of +/- 2%; however this is much smaller than the change in volume of property transactions. The seasonal effect is estimated twice a year using established statistical methods.

For the monthly house price index where changes can be as little as 0.1%, seasonal factors are important. The Nationwide therefore produce a seasonally adjusted series for UK house prices which seeks to remove this effect so that the overall trend in prices is more readily apparent.

Seasonal adjustment shows that June is generally the strongest month for house prices (raw prices are 1.3% above their SA level) and January is the weakest (raw prices are 1.9% below their SA level).

Moody's Analytics supplement

For the monthly house price, we construct a seasonally adjusted counterpart.

Further reading

At the source:

• http://www.nationwide.co.uk/hpi/methodology.htm

At third parties:

• Building Societies Association
• ADV Ratings - Table: 2022 top 10 building societies, assets, membership, branches
• TheBanks.eu - Table: 2019 all 43 building societies

About the source

Nationwide is a building society founded in 1884 and updated to its current name in 1970. Under British law, a building society is a customer-owned cooperative financial organization, a.k.a. a mutual financial institution, whose core business is mortgage lending. As of 2022 there are 43 such entities, with 25 million customers.

• 26 Jan 2023, Phillip Thorne - Moody's Analytics supplements