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HMRC internal manual

VAT Relief for Disabled People Manual

Motor vehicles [items 2(f) and 2A]: vehicle conversion after the initial supply

A business may supply a standard production vehicle to a disabled person or to an appropriate charity, but with an arrangement to convert the vehicle to meet the criteria under Item 2(f) or Item 2A at a later stage.

If this applies, then only the chargeable conversion is eligible for zero-rating. This is because the title has changed hands, a supply of goods has taken place, and so only adaptation services can be performed.