Stamp duty's effective date: when the dwelling count is fixed

Whether a block of flats is taxed as six dwellings or five can move a stamp duty bill by around £30,500 on a £600,000 purchase. What is easy to miss is that the count is not settled when contracts are drawn up, when planning permission comes through, or when the sales brochure is printed. It is fixed on a single day — the transaction's effective date — and judged on the physical state of the building as it actually stands that day. A sixth flat whose kitchen has not been fitted by that date is, for stamp duty land tax (SDLT), not yet a dwelling.

This is the timing half of the six-dwelling rule. How HMRC counts a separate dwelling — the facilities, access and privacy tests — decides what is a dwelling. The effective date decides when you take the photograph.

The rule in one line

Section 116 of the Finance Act 2003 defines residential property, and section 116(7) provides that where six or more separate dwellings are bought in a single transaction, they are treated as not residential — so the purchase falls under the flatter non-residential rates. Section 119 fixes when that test is applied: the "effective date" of a land transaction. And a long line of tax tribunal decisions confirms the same principle — a property's status is judged on its physical condition at the effective date, not on what it is intended to become.

So three questions have to be answered in order: what is the effective date, what did the building look like on that date, and how many separate dwellings did that physical state contain.

What the "effective date" actually is

Under section 119, the effective date is normally the date of completion — the day the money changes hands and the keys transfer. But section 44 can pull that date earlier. A contract is "substantially performed" — and the effective date jumps to that earlier moment — when either of two things happens first:

• the buyer takes possession of the whole or substantially the whole of the property (going into occupation, starting works, or becoming entitled to receive the rents), or
• a substantial amount of the consideration is paid. HMRC treats "substantial" as around 90% or more of the total price.

Whichever comes first — substantial performance or completion — sets the effective date. For most straightforward purchases the two coincide and the distinction never matters. For a part-built or part-converted block it can matter a great deal, because it decides which day's physical state is the one that counts.

The state of the building, not the plans

The tribunals have been consistent: SDLT looks at the property as it physically exists on the effective date, not at its planned or intended end-state. In P N Bewley Ltd v HMRC [2019] UKFTT 65 (TC), a bungalow so dilapidated it was not "suitable for use as a dwelling" on the day of purchase was taxed at the non-residential rates — an intention to refurbish it did not make it a dwelling on that date. In Ladson Preston Ltd v HMRC [2022] UKUT 301 (TCC), the Upper Tribunal held that the conditions for relief had to be met at the effective date; planning permission obtained afterwards could not be read back to change the position.

The read-across to the six-dwelling count is direct. Drawings showing six self-contained flats do not make six dwellings. Six dwellings exist only when six units are each — on the effective date — suitable for use as a single dwelling: their own kitchen and bathroom, their own lockable access, their own privacy. Miss one of those on one unit, and the count is five.

The £600,000 worked example

Take an investor buying a small block for £600,000 to let out. The building is designed as six flats. On completion day, five are finished; the sixth is complete except that its kitchen has not yet been installed.

Because a functioning kitchen is one of the facilities that marks out a separate dwelling, that sixth unit is not — on the effective date — a dwelling. The block is bought as five dwellings, not six. That flips the whole transaction from the non-residential rates to the residential rates, and because the buyer already owns other property and is acquiring more than one dwelling, the 5% additional-property surcharge applies to the whole price.

SDLT figures calculated on Homecost's stamp duty engine for England, 22 July 2026. The non-residential leg is checked against HMRC's published non-residential slabs: 0% to £150,000, 2% to £250,000, 5% above.

The £30,500 gap is almost entirely the 5% surcharge that the residential route carries and the non-residential route escapes. All three residential surcharges — the 5% for additional dwellings, the 2% for non-UK residents, and the 17% flat rate for companies buying dwellings over £500,000 — are residential-only charges. A purchase taxed under the non-residential table sits outside every one of them. One unfitted kitchen, on one particular day, is the difference.

Timing cuts both ways

Because the effective date can be pulled forward by substantial performance, when a buyer takes control of a part-finished block matters.

Take possession early — to start the final fit-out, say, or to begin letting the finished flats — and the contract may be substantially performed before the sixth kitchen goes in. The effective date freezes on that earlier, less-complete day, locking in a count of five. Reach genuine completion after the sixth unit is finished, and the snapshot is taken with all six dwellings in place. The same building, the same price, two different effective dates, two very different bills.

The principle runs in the other direction too. A large house being converted back into a single home from flats is judged on its state on the day: mid-conversion, it may still be several units; once the works are done, one. Neither the starting point nor the finished intention governs — only the physical reality on the effective date.

The stakes scale with price

The size of the swing grows with the purchase price, because more of the price is dragged into the residential 5% and 10% bands while the non-residential scale stays flat and the surcharge is a flat 5% of the whole:

Residential legs from Homecost's stamp duty engine (England, additional-property rates, 22 July 2026); non-residential legs computed on HMRC's published slabs.

These are not fringe numbers. Of the 759,637 homes sold across England and Wales in 2025 (HM Land Registry price paid data, standard sales, figures fetched 22 July 2026), 141,931 — around one in five — changed hands at £500,000 or more, and 92,641 at £600,000 or more. Block and portfolio purchases sit at the upper end of that range, where a single-day classification question is worth tens of thousands.

When the clock starts

The effective date does more than fix the dwelling count. It also starts the SDLT clock: a land transaction return and any tax due must be filed within 14 days of the effective date (section 76, Finance Act 2003). Since Multiple Dwellings Relief — the old averaging relief for bulk residential purchases — was abolished for transactions with an effective date on or after 1 June 2024, the six-dwelling route into non-residential rates has become the main remaining mechanism that softens a bulk buyer's bill. Our guide to what the abolition of Multiple Dwellings Relief changed for bulk buyers sets out the wider picture, and how the mixed-use and six-dwelling routes into non-residential rates compare explains the two ways a residential-looking purchase can end up on the non-residential table.

See the numbers for your own case

You can model the residential legs of any of these examples on Homecost's stamp duty calculator, which covers all four UK regimes and the additional-property, first-time-buyer and non-resident rules. For a sense of the blocks-of-flats market these purchases come from, the Manchester city-centre flat market is a natural starting point, and you can browse more worked explainers in Cost Intelligence.

Whether a specific unit counts as a dwelling on a specific day turns on fine facts, and the effective date can itself be disputed where possession and completion diverge. This is general information about how the rules work, not advice. Speak to a qualified adviser before acting.