Users of UK Finance's monthly Mortgage Trends data will have noticed some changes in today's release. Although we?re confident that these changes enhance how we inform our members and industry observers on how the mortgage market is moving, we wanted to take this opportunity to explain what's new. So, we set out here the what, why and how of the changes we?ve made.

Evolution, not revolution

The structure of our monthly mortgage figures hasn't materially changed for over a decade when the mortgage market was a very different proposition to the one we see today. A facelift was in order to allow us to match the realities of today's market with the most relevant data.

For those of you who value continuity, don't worry - we haven't stopped producing anything that we did previously. Rather, we?ve tweaked our methodology in some perhaps dull but important ways, and we?ve added some new information that allows us to report and comment more usefully on today's market.

Deeper insight into remortgage

Last year we saw the highest number of remortgages in a decade. And as we noted in a blog last year, remortgaging is set to grow further in 2019. But there are varied reasons why borrowers look to remortgage. These have different affordability considerations, as well as different wider market implications when looking at aggregate trends.

Our new-look release lets users differentiate between simple refinance transactions - where borrowers are just switching mortgage lender in order to get a better deal - and remortgages where borrowers withdraw equity at the same time.

Taken together with our separate quarterly figures on Product Transfers, which UK Finance started to publish from the beginning of 2018, we can see that some 1.4 million borrowers switched mortgage just to move to a better deal last year, with a further 220,000 or so taking out equity at the same time. Altogether that's almost one in five of all mortgaged homeowners and, although the stock of ?free-to-move? borrowers is a constantly moving target, it also represents half of the number who were not on deal rates at the start of 2018 (and so are free to move without incurring early repayment charges).

These figures are strong evidence of a mortgage market where customer engagement is high - not just at the outset, but through the life of the mortgage and where lenders provide attractive refinancing options to suit borrower needs.

With this year set to be an even stronger one for refinancing, our data now shows the same detailed affordability figures for remortgaging as we have always shown for house purchase mortgages. Users of our data can therefore now monitor ongoing mortgage affordability, not just at the point of purchase.

New basis for affordability statistics

(Brace yourselves, this is the dull bit).

We?ve changed the basis of how we present average statistics - loan-to-value (LTV), income multiples, age of borrowers and so on - from median to means because means are more responsive to changes in the data. That is, when the data starts to move in a different direction, the mean is quicker to respond and show this trend. The median tends to remain rooted to the prevailing market ?norm? for longer, even if there's an underlying shift away from this norm taking place.

Moving from medians to means also means that the absolute values of affordability figures shifts, although the trends remain the same. This doesn't mean that either means or medians are wrong, just that they are different presentations of 'typical? characteristics.

The chart below gives an example of this, showing the difference between mean and median LTV. Whilst both show the same underlying trend, the mean is lower but moves more smoothly - the median has larger but less frequent jumps, giving a less timely and sometimes less meaningful reading of changes in lending.

Chart 1. First-time buyer loans: mean vs. median LTV

(Source UK Finance Regulated Mortgage Survey)

Improved base estimate of market size

Finally, we have incorporated external information newly available to us to refine the grossing methodology we use to estimate total market volumes from our sample of reporting lenders (currently well over 90 per cent of total new lending).

This means that our historic figures have been revised downwards slightly. For consistency we?ve rebased back to 2005, when our current data source begins.

This rebasing doesn't change any trends or key findings, see chart two below on first-time buyer loan volumes. However, users should note the differences and refer to our new data series for the most accurate market view.

Chart 2. First-time buyer loans new (rebased) volumes vs previously published

(Source UK Finance Regulated Mortgage Survey)

Better placed to monitoring the market in an uncertain environment

While change is sometimes unwelcome, we think it's the right time to make these improvements.  Even in this time of huge national uncertainties, the residential mortgage market is holding up relatively well at present, but it's unclear how long this resilience will continue should the wider economic conditions deteriorate. Should they do so, these changes will allow us to better identify where and how any inflexion point might occur.