Your commission is standard-rated. It is not exempt.
Schedule 9 Group 1 exempts the grant of an interest in or right over land, and an agent grants no such interest — it supplies services. HMRC treats the engagement as a single supply of services normally to the seller, with a basic tax point when the work for the principal is completed. The finance exemption in Group 5 does not rescue it either: item 5 covers intermediary services in relation to financial transactions, and a land sale is not one.
So the headline percentage is not what you keep. On a VAT-inclusive fee, a sixth of it was never yours. This calculator starts there, because it is where the conversation with a vendor usually starts too.
Referral and portal costs are usually recharges, not disbursements
HMRC's eight conditions, at VTAXPER39000, must all be met for a payment to be a true disbursement — including condition two, that the client and not the agent used the goods or services. Revenue and Customs Brief 6 of 2020, following the Brabners case, sharpened it: a fee is not a disbursement where you use the output in making your own supply, because it is then a cost component of that supply.
Portal advertising fails immediately — Rightmove supplies you, not the seller. Tenant referencing is directly analogous to Brabners, because you use the report to form your own recommendation. An EPC can qualify, but only where the assessor contracts with the seller, the seller is responsible for payment, you pay as authorised agent and the exact sum is itemised. Buy it in your own name and it is a recharge. HMRC has published no estate-agency-specific ruling on portal costs or referencing, so treat those two as reasoned analysis rather than as HMRC's stated position.
The point of the exercise
To have the net figure per instruction in your head before you discount. A percentage point off the fee is a much larger percentage off the margin than most agents assume, particularly once a referral fee is in the deal.