The government's planned high-value council tax surcharge faces a significant implementation challenge: nearly 40% of properties estimated above £1.5mn lack recorded sales data, making accurate valuations difficult. This shortage of comparable transactions threatens the £430mn annual revenue target from the tax, set to begin in April 2028.

Zoopla research shows 183,000 homes valued above £2mn, concentrated heavily in London and the South East. For these £1.5mn-plus properties with sales history, the median time since last transaction reaches 11 years. Almost two-thirds lack energy performance certificates, which increasingly affect valuations. Property characteristics like basements or renovations often remain invisible without physical inspections.

Richard Donnell of Zoopla highlights the complexity in valuing the top 1% of the market, where properties differ significantly and transactions occur infrequently. The Valuation Office Agency will assess properties at the £1.5mn threshold using automated models supplemented by professional judgment. However, features hidden from street view create assessment gaps that automated systems cannot bridge.

London alone contains 125,000 qualifying homes, with the South East adding another 34,100, while the North East has just 200. The government's consultation on exemptions and challenge procedures closes July 14, as officials acknowledge combining automated models with human expertise will be essential for accurate implementation.