SolvedCA11150 | General: claims: tax agreements
From HM Revenue & Customs · Capital Allowances Manual
A person’s capital allowance claim may be agreed without an assessment being made. This is unlikely to be met in practice. But it could happen with, for example, a PAYE taxpayer.
A person who is entitled to an allowance for a year of assessment may make a written agreement with the inspector about the extent to which the allowance is to be given effect for that year. If so the allowance is deemed to have been made for that year even if no assessment is made.
If there is no written agreement you should make an assessment. Where no assessment is made you cannot treat the allowance as made unless there is a written agreement.