An office block in Mayfair avoided thousands of pounds in business rates by failing to declare a hidden sixth floor used as workspace.
The hidden storey had been added without planning permission but was concealed from street view.
The property was registered for rates from the basement to the fifth floor, but officials discovered another secret storey from studying Google Maps satellite view.
An unannounced visit by a council revenue inspector uncovered office space on the sixth floor on both sides of the lift lobby.
The undeclared space was worth £38,572.50 in business rates annually. The business was charged a total of £112,103.50 in taxes backdated to 2023.
A retrospective planning application has been submitted to use the sixth floor as commercial or residential space.
Westminster City Council wrote off £34million in unpaid business rates in 2023/24, by far the largest contributor to the local authority’s £54million of irrevocable debt that year.
More than £400,000 was also lost last year to fraud, the bulk of which was from scams relating to council tenancies.
Among cases detailed in a report to the audit and performance committee were several instances of tenants subletting council properties.
They include man who rented out a studio flat on Ebury Bridge Road while living in Haringey and taking a monthly revenue of up to £800 letting it out.
He refused to attend an interview or accommodate a visit, but surrendered the keys to the property just as legal action was being pursued and before the case went to court.