hi Gill, you guess it right: in Italy we have a list of coefficients to adjust the selling price according to the specific conditions of the real estate under consideration. See here an example https://www.borsino-immobiliare.com/pagina/coefficiente-diff... .
It comes from the fact that our Internal Revenue Services constantly monitor the sales of real estate and quarterly release the average price per-square-meter in any given place in Italy, (average market value, i.e. valore mercantile!). below you van find the link to Agenzia delle Entrate - Banca dati delle quotazioni immobiliari !
In a specific sales you then have to adjust, to a certain extent, this avg price to reflect the real value of your piece of property. So for example, if 100 is the avg house price, an apartment of the ground floor will be 80 while a penthouse will be 120. And so on.
Clearly anyone is free to choose the price they want when selling their house, bu a too large deviation from the values obtained applying the coefficenti mercantili (it's market coefficients, BTW) will proably result in a no sale.
I am not aware of a similar system in either England or US, while I know for a fact Home Appraisals are a common thing (there are even professionals for that). Hence my suggestion for translation here above.