When people talk about house prices, they usually mean what a particular flat sold for. A house price index is something different: a single number that tracks how prices in a defined area move over time. Reading one correctly takes a few ideas, and avoids a lot of confusion.

What an index number is

An index sets a base period equal to 100. If a city’s index is 120 now, prices are 20% higher than in the base period, on average and for the kinds of transactions the index covers. It says nothing about the price of any individual home, and the level has no meaning without the base year.

Change versus level

The numbers that usually matter are changes: from the previous quarter, or from the same quarter a year earlier. As with company results, year-on-year changes strip out seasonal patterns. A city moving from 120 to 126 over a year has a 5% annual change (6 ÷ 120).

Things that affect what an index says

  • Which cities, which kinds of properties and which transactions are included. An index built from registered sale deeds differs from one built from bank valuations or from advertised asking prices.
  • Some indices adjust for the size and quality of homes; others do not. Mix shifts, such as more large flats being sold in a quarter, can move a simple average without any change in underlying prices.
  • Data often arrives with a lag of several months. Always look at the quarter the figure refers to.
  • Nominal versus real. Index changes are generally nominal. To judge purchasing power, compare with general price inflation over the same period.

A fictional illustration

In a made-up city, the index is 140 in the first quarter of one year and 147 in the first quarter of the next: a 5% change. Over the same year, consumer prices rose 6%. In nominal terms house prices rose; in real terms they were roughly flat to slightly lower. Both statements describe the same data.

Questions to ask of any housing number

  • Which source, and which quarter?
  • Is it a level, a change, or an average price?
  • Does it cover your city and the type of home?
  • Is the change nominal or real?

Index, median price, or price per square foot?

These are three different measures, and news reports sometimes use them interchangeably.

  • An index tracks change over time against a base. It is designed to be comparable from one period to the next.
  • A median price is the middle transaction price in a period. It moves when the mix of homes sold changes, even if no individual home changes in price.
  • Price per square foot depends on how area is measured (carpet, built-up, super built-up) and on the location within a city.

A report saying “prices rose 8%” could mean any of the three. Always find which one, and for which area.

Why two sources can disagree

Suppose one source reports a 4% annual change for a city and another reports 9%. Common reasons include different data (registered transactions versus bank valuations versus listings), different coverage (older homes only, or new launches only), different methods (adjusting for quality or not), and different periods. Neither need be wrong. The useful response is to read the methodology notes and compare like with like, not to pick the one you prefer.

Writing about the data neutrally

The temptation with housing figures is to attach a view: “prices are soaring” or “the market is cooling”. An index does not say that. It says that a measured value changed by a certain amount between two dates. A neutral sentence reads: “The index for the city was 147 in the first quarter, compared with 140 a year earlier, a change of 5%, according to [source].” It names the figure, the comparison and the source, and leaves interpretation to the reader.

A cross-checking routine

  • Note the source, the quarter and the base year.
  • Record the level and the year-on-year change.
  • Check inflation for the same period, to see the real change.
  • Compare the city with others using the same index, not different ones.
  • Look for a note about revisions. Early estimates are sometimes updated later.

Limits

An index does not tell you what to pay, whether to buy or sell, or what will happen next. It cannot reflect the particular features of a home or its street. For personal decisions, local information such as recent sales nearby, the condition of the property and legal checks on the title matter far more than a city-level number. Other examples of figures that need a second look are collected in the Reality corner section of wealthstar.in.

The main value of a well-presented index is that it lets many people look at the same figures, with the same source and date, and discuss them on common ground.

Where to find the data

The best starting point is always the official source and its methodology note. For a state-by-city view that puts the quarter and source next to every figure, see a state-by-city view of the official house price index, with the quarter and source beside each figure, which is organised for exactly this kind of cross-checking.

An index is a summary of many sales over a period. It can inform your understanding of a market, but it is not a quote for any home, and it is not a forecast.