Bring sales, rents, and build-cost evidence into assessments

Use market evidence—sales, rents, construction costs, or land values—to anchor assessments more credibly. Even if the city cannot yet run advanced models, better evidence helps move valuations closer to current realities and makes them easier to defend.

• Some usable market data exist but are not yet feeding the tax system.

• The city wants a stronger evidence base before a wider revaluation.

• Valuation disputes are partly driven by the sense that the numbers are detached from reality.

• Access to at least one credible source of sales, rent, or cost data.

• A team or partner who can clean, test, and interpret that evidence.

• Do not overstate data quality; weak or biased market data can damage credibility.

• This is not the first move where the city has no workable property register to attach evidence to.

Valuation reforms are usually easier to defend when they are presented as fairness and credibility measures, not simply as ways to raise more money. Sudden unexplained changes in bills tend to provoke resistance.

The full card would help the city plan how to gather and clean market evidence, decide which proxies are usable, and translate that evidence into a more defensible assessment process.

• Revalue first where the money and change are concentrated

• Move to mass valuation when data and capacity allow.

Assessment accuracy improves when values are anchored in observable evidence rather than in inherited assumptions alone. That evidence can come from sales, rents, declared transaction values, construction costs, land values, or other market proxies. The city does not need a perfect market database before starting. But it does need to know which sources are usable, how biased they may be, and how to turn them into a consistent input for assessment rather than a pile of inconsistent anecdotes.

• Some market evidence exists but is not yet feeding the tax system in a systematic way.

• Valuation disputes are partly driven by the sense that current figures are detached from reality.

• The city wants a stronger evidence base before wider revaluation or method reform.

• Start by mapping all plausible evidence sources and grading them by reliability, coverage, and ease of access rather than assuming official data are always best.

• If transfer values are clearly under-declared or rents are distorted, use them cautiously and combine them with build-cost or zonal information rather than discarding all evidence.

• Begin with the strongest available source or neighbourhood rather than waiting for a perfect integrated market dataset.

• Which evidence sources the city will trust most in the first phase: sales, rents, build costs, contractor schedules, land values, or a mix.

• Whether the city will aim first for better zonal/value tables, better manual assessment decisions, or inputs for later mass valuation.

• How the city will clean, test, and document evidence so later users can understand where the numbers came from.

First 90 days

• Identify all potential evidence providers, including registries, notaries, planning departments, major brokers, builders, banks, and internal permit records.

• Review access conditions, biases, and data fields for each source and choose the first dataset or datasets to test.

• Define the minimum cleaning and documentation process so raw evidence can be turned into something usable for assessment.

6 to 12 months

• Collect and clean a first round of data and test how well it explains known property differences across areas or classes.

• Use the evidence to refine land-value zones, build-cost tables, or pilot assessments in a limited area.

• Document where the evidence is strong, weak, seasonal, or systematically biased.

12 to 24 months and beyond

• Institutionalise data refresh arrangements with the strongest sources so evidence gathering does not depend on one-off relationships.

• Bring the evidence into recurring revaluation, band review, or quality-control processes rather than leaving it as a pilot dataset.

• Use the growing evidence base to decide whether the city is ready for more advanced mass valuation tools.

• Clarify whether current law allows the city to use the chosen data sources for tax purposes and what privacy or confidentiality rules apply.

• Where partner data are needed, formalise at least the key access and use conditions so the flow is not purely personal or informal.

• Make sure the assessment record can show the type of evidence used at an appropriately aggregated level without exposing protected individual information.

• The city needs at least one team or partner with the ability to clean, interpret, and test evidence rather than merely collect it.

• Valuation staff and IT or GIS staff should work together early if the evidence will later feed zonal tables or models.

• Management should insist on documentation standards so future staff understand the source and quality of the evidence.

• Weak or biased data can damage confidence if presented as more authoritative than they really are.

• If data cleaning becomes too ambitious, the city can spend months assembling evidence without using it to improve any actual assessments.

• If the city adopts evidence from one neighbourhood or market segment and generalises it too widely, fairness can worsen rather than improve.

• Number of usable records or observations by evidence source.

• Coverage of evidence across the main neighbourhoods or property classes the city wants to assess better.

• Changes made to valuation tables, zones, or pilot assessments using the new evidence.

• Frequency with which valuation staff actually use the evidence set in live assessment work.

• Revalue first where the money and change are concentrated.

• Move to mass valuation when data and capacity allow.

• Tighten valuation data and quality control.

• Which evidence source is strong enough to start with right now?

• What are the biggest known biases in local sales, rent, or cost data?

• Who will clean and interpret the data once it is collected?

• How will the city turn evidence into better assessments rather than just more files?