Ah, yes. So, it’s a bit complicated.
As far as I am aware, the portal works on what price the house last sold at and adjusts it for inflation.
I don’t think .io takes in surges in house prices if lots of expensive houses have sold nearby.
For example, Zoopla takes in all local house sales which can spike the value. Say you have a house that’s worth 200k and within the last year four mansions sold nearby, Zoopla might spike your house value to 400k. Which is silly, cos you are in a two bed worth 200k.
So to gauge how accurate io is, I take the following steps:
I look at when the house last sold. There have been several house price spikes over the last 20 years and as a general rule, people overpaid at those 3 or 4 peaks.
I look at the io value for identical houses bought in non-peak years.
I dig out the old sales adverts on Rightmove and Zoopla to see if they have added value, for example by doing a complete refurb, adding an extra 25% floor space, etc.
I look at local planning portals which tells me things like; gas boiler installed, 6 windows installed, (electric) consumer unit installed, all of which add value.
I then put the postcode into the nationwide house price index from land registry’s price paid, and for the “time it was last valued” put in approximately six months before its last completion. This index is based on mortgages they gave out and as they are a very large lender, it’s somewhat reliable. Because their index is based on actual mortgages handed out, it also includes house price spikes because that’s what people paid at the time of purchase.
I take all of these things into account. Last week I looked at one paricular street with absolutely identical houses all purchased over various peaks and normal times in the space of about 20 years, all in exactly the same condition with no floor space added, the io value varied by as much as around 25% depending on which year the house was bought.
If they bought in a spike, they generally overpaid.
You can come to an approximate value when you accumulate all of this information. But you will probably find that most house prices are priced bang on the median price listed on Zoopla, because EAs are lazy, and most people think Zoopla is right, when it’s often wildly inaccurate. It dropped the value of my rental from around 600k to around 270 and then back up to around 530 all the space of 3 months because some cheap houses sold around me. Actual value is probably 550 to 600. Don’t listen to Zoopla.
I hope that helps. If you need further help, feel free to PM me your email address and I will send you the above mentioned street example, and help further if I can.