Related answers
An end user is a customer who receives the construction services for their own purposes rather than to sell them on as part of a further construction supply. A property owner having work done on their own building is the clearest case. A main contractor buying your labour to deliver their own contract is not.
An intermediary supplier is connected to the end user, or shares an interest in the same land, and buys and re-sells construction services without materially altering them. They are treated the same way.
Whether your customer is an end user depends on facts about their business you have no way of seeing from your side of the invoice. That is precisely why the rules put the obligation on them to tell you, in writing, and why the reverse charge is the default until they do.
A verbal assurance on site, an assumption based on what the building looks like, or the fact that you have always charged VAT to that customer are not notifications.
It should identify the customer, state clearly that they are an end user or an intermediary supplier for the purposes of the domestic reverse charge, and be held on file. It can sit in a contract, a purchase order or an email. What matters is that it is in writing, it is specific, and you can produce it.
Practically: go through your regular customers and establish who has given you one. Most businesses that have been getting this wrong find the problem is not a judgement call gone astray but a notification nobody ever asked for.
This question is part of The VAT domestic reverse charge, which covers the whole area.
Send us three months of payment and deduction statements. We will tell you what is wrong, what it is costing, and whether gross payment status would fix it.