How model assisted valuation delivers large-scale property valuations
Source: gov.uk (2025-03-30)

How the Valuation Office Agency uses model assisted valuation and mass appraisal to support large-scale valuation.

Overview

When undertaking a large-scale valuation exercise, mass appraisal is a modern, efficient and cost-effective way of providing consistent and accurate valuations.

Mass appraisal is the valuation of a group of properties using common data, standardised methods and statistical testing.

Mass appraisal is commonly used in countries that base taxes on the value of property. It is also used in the private sector, for example, most initial mortgage valuations are from mass appraisal models.

An Automated Valuation Model, or AVM, is a key element of mass appraisal.

Similarly to valuers, AVMs use data about sold properties, including their sales price, property characteristics and location. Whilst valuers typically focus on a small group of comparable properties at a time, AVMs use algorithms and statistical techniques to produce estimates of property’s value at a set point in time for all properties.

An AVM provides a fair and consistent basis for a valuation, because all properties with the same characteristics will be valued in the same way.

Currently the VOA is using mass appraisal to support Welsh Government’s Council Tax reform agenda.

How the model is being used

The VOA has adopted a model assisted valuation approach, to help target its valuer resource when valuing properties at scale.

The model is being used to support the valuation of 1.5m domestic properties for a Council Tax revaluation in Wales in 2028.

VOA valuers underpin each stage of the model assisted approach.

First, they ensure the property data and sales used are accurate. They then collaborate with analysts to build and refine the AVM.

Valuers sample model valuations to ensure the model is accurate and to understand where valuer reviews should be targeted.

Groups of properties, with similar characteristics in the same location, are then reviewed. The focus of valuers is on reaching the right Council Tax band for each property.

There is a minimum and maximum property value for each band and valuers will also focus on checking those properties at the band margins to ensure a property is placed in the correct Council Tax band.

Additional checks are made on any valuations that stand out in some way. This could include properties that have seen a large change in band, or properties with similar characteristics in the same street which are in different bands.

Customers will have a legal right to challenge their band if they disagree.

External assessment of the model

The AVM has been assured externally:

The Centre for Appraisal Research and Technology (CART), experts in mass appraisal, provided advice during the model development.

The VOA commissioned the International Association of Assessing Officers (IAAO), as a recognised world authority on AVMs, to carry out a review of the model development process.

The IAAO’s report on the model concluded:

“These findings are more than satisfactory and should lead the VOA to have confidence in the quality of the new valuation project conducted in Wales.”

The VOA has published the reports (https://www.gov.uk/government/publications/external-assurance-of-the-voa-automated-valuation-model-avm-voa-response) from both external experts and the actions we are taking.

Further information

More information on how the VOA is using mass appraisal to support Council Tax reform in Wales and on the AVM (https://assets.publishing.service.gov.uk/media/67e524ddbb6002588a90d5de/More_information_on_mass_appraisal_and_AVM.pdf) is available.

The VOA has also published the model specification (https://assets.publishing.service.gov.uk/media/67e522a487cebda7c4ca4c32/Model_specification_document.pdf).