No sooner had I written the article "Do house prices double every 10 years?" than I get the question posted on LinkedIn as to whether house prices quadruple every 20 years?

My first reaction was - No Way, if they double in 10 years, how could they possibly quadruple in 20 years? - to be clear $100 doubling in 10 years becomes $200 and within 20 years quadrupling to $400. But then I remembered the good old adage of compound interest. The annual rate of growth to double a sum of money in 10 years is 7.2% and that same 7.2% annual growth rate compounded will quadruple the original sum within a 20 year timespan.

The next question for me was to Google the question, just as I did with doubling house prices, not 339 million results this time just 8 million. But top of the results from The Telegraph in the UK with a 2003 article stating just that fact. Between 1983 and 2003 average house prices went from £29,993 to £121,742 – a quadrupling. As an aside I bought my first house in the UK in 1984 for £22,150 so I can confirm that is what a typical house cost back then. I sold that house in 1985 for £20,650 so clearly I should have waited!

Anyway I digress. Back to the question posed on LinkedIn. Given the fact that NZ property prices have doubled during some 10 year periods, have we seen quadrupling over any 20 year period?

The answer is yes for Auckland but not for the rest of the country in aggregate. I have only been able to assess the period from 2012 as the available data from REINZ of sale price data only goes back to 1992. Here then are the same charts I used in the prior article adapted to track each 20 year period from 2012 to 2018.

## NEW ZEALAND - ALL PROPERTIES

So for the period from 2012 onwards the 20 year rise in house prices across all of NZ was an average of 237%, that is over a three fold increase but at no time has a quadrupling been experienced. Of note is the fact that the current period June 1998 to June 2018 at 271% is the highest ever 20 year rise.

## AUCKLAND

In the case of Auckland the average 20 year period price rise has been 296% between 2012 and 2018. In 31 of the 78 months the 20 year rise was more than quadrupling of prices in that 20 year period. The highest ever rise was for the period from August 1995 to August 2015 which saw a 329% increase.

## NEW ZEALAND EXCLUDING AUCKLAND

For the rest of NZ excluding Auckland the average 20 year price growth has been 197%, so just under a trebling of prices over a 20 year period. Of note though is the steady rise in the price rises since 2015 with the latest month of June recording a 240% increase in prices since June 1998.