Can a Linked Annex Still Qualify as a Separate Dwelling for SDLT Multiple Dwellings Relief?

Introduction

Buyers often ask whether an annex, granny flat or self-contained part of a house can count as a separate dwelling for Stamp Duty Land Tax purposes. This matters because, where a purchase includes more than one dwelling, Multiple Dwellings Relief (MDR) was historically available under the Finance Act 2003. Although MDR has now been abolished for most new transactions, disputes still arise in older cases and in HMRC compliance checks about whether relief was validly claimed at the time.

A common problem is where the annex has its own facilities but is physically linked to the main house by an internal door. HMRC may argue that the whole property is really one dwelling. The legal position is more nuanced than that. The question is not simply whether there is a connecting door. The real issue is whether, applying the legislation and case law, the annex was suitable for use as a single dwelling at the relevant date.

The Question

A couple bought a property consisting of a main house and an annex. They claimed MDR on the basis that the purchase included two dwellings. HMRC later challenged the claim and said a later “triggered event” meant the relief had to be repaid with interest.

The dispute focused on a lockable internal fire door between the main house and the annex. The buyers said this was not installed to merge the two units into one home. Instead, they said works were needed because defective construction was discovered and the opening had to be properly formed and made compliant for structural and fire safety reasons. They also relied on plans and other evidence showing the annex had separate living accommodation.

The practical question was whether the annex could still count as a separate dwelling for MDR purposes despite the connecting door and later remedial works.

Nick’s Explanation

Nick’s core view was that HMRC were putting too much weight on the existence of the lockable door and not enough weight on the statutory test. In anonymised form, his explanation was:

“The law looks at whether the annex was suitable for use as a separate dwelling at the date of purchase. A later fire door added for safety or compliance reasons does not, by itself, change that. If the annex remained independently usable, it can still count as a dwelling.”

He also identified the key legal points:

• the relevant definition of residential property is in Finance Act 2003, section 116;
• a “building” includes part of a building, so an annex can qualify in principle;
• for MDR, the question is whether each unit is suitable for use as a single dwelling;
• HMRC guidance is not the law, and the tribunal or court will apply the legislation and authorities;
• the objective condition of the property matters more than informal descriptions used in correspondence.

Nick also took the view that, if HMRC were relying on the idea that the number of dwellings had later been reduced, there was an argument that remedial and compliance works do not amount to a deliberate conversion into a single dwelling where the annex remains capable of independent occupation.

The Law

The starting point is the Finance Act 2003.

Section 116(1)(a) provides that residential property includes “a building that is used or suitable for use as a dwelling”. Section 116(6) makes clear that “building” includes part of a building. That means a self-contained annex or subsidiary unit can, in principle, be a dwelling even if it forms part of a larger structure.

MDR was contained in Finance Act 2003, section 58D and Schedule 6B. The relief applied where a chargeable transaction included an interest in at least two dwellings. Paragraph 7 of Schedule 6B dealt with the meaning of “dwelling” for these purposes and focused on whether the property, or part of it, was suitable for use as a single dwelling.

In practice, tribunals and courts have looked at matters such as:

• whether the annex had the basic facilities for day-to-day living, such as washing, sleeping, cooking and toilet facilities;
• whether it had a sufficient degree of privacy and independence;
• whether it could realistically be occupied as a home on its own;
• whether any physical link to the main house prevented it from being a separate dwelling.

Where a taxpayer argues that a property was not suitable for use as a dwelling because it was derelict or uninhabitable, the threshold is now relatively high following Amarjeet and Tajinder Mudan v The Commissioners for HMRC [2025] EWCA Civ 799. That case confirms that “not suitable for use” is not a low bar. Serious disrepair or inconvenience will not automatically be enough. Although that authority concerns suitability for use as a dwelling more generally, it is relevant because it shows the courts are likely to apply a practical but fairly demanding test.

Analysis

The issue can be analysed in stages.

First, was there an annex or subsidiary unit capable in principle of being a dwelling? If the annex had its own living accommodation and facilities, that points towards separate dwelling status. The fact it formed part of the same overall building does not prevent that, because section 116(6) expressly allows part of a building to qualify.

Second, does an interconnecting door automatically mean there is only one dwelling? Usually, no. A connecting door is relevant, but it is not conclusive. Many properties contain linked accommodation that is still capable of separate occupation. The legal test is suitability for use as a single dwelling, not whether there is total physical separation in every respect.

Third, what is the significance of the lockable fire door? On the facts described, the better argument is that the door was a compliance and safety measure connected with remedial works, not evidence that the annex ceased to be a separate dwelling. If the opening already existed historically, or if the works simply regularised a defective structure and inserted a compliant door, that is different from a deliberate redesign to absorb the annex permanently into the main house.

Fourth, what date matters? For the original MDR claim, the key question is the condition and suitability of the property at the effective date of the transaction. If HMRC are instead relying on a later clawback or “trigger” provision, the later changes must be examined carefully. The question would then be whether, within the relevant statutory period, there was in substance a reduction in the number of dwellings. If the annex remained independently usable after the works, HMRC may struggle to show that the number of dwellings truly fell from two to one.

Fifth, how strong is HMRC’s reliance on correspondence? Usually, limited. Statements made by homeowners in emails can be relevant evidence, but they are not decisive if they do not accurately describe the legal or physical position. Plans, surveys, contractor evidence, photographs, building control material and the actual layout of the annex may all carry more weight.

Sixth, does the “not suitable for use” line help? Possibly, but only in the right case. If part of the dispute is whether defects meant the annex was not suitable for use as a dwelling at some stage, the court’s approach after Mudan suggests that substantial evidence is needed. Minor defects, non-compliance, inconvenience, or the need for upgrading will not necessarily mean a unit was not suitable for use. So if the taxpayer’s case is that the annex remained a dwelling despite defects being discovered and repaired, that may sit more comfortably with the modern authorities than arguing it was incapable of being a dwelling at all.

Outcome

The practical conclusion is that a lockable interconnecting door does not automatically prevent an annex from being a separate dwelling for SDLT purposes. If the annex retained the features of independent residential occupation, there is a credible argument that MDR was correctly claimed.

On the facts described, the stronger analysis is that remedial structural works and the installation or re-formation of a compliant fire door do not, by themselves, show that two dwellings became one. HMRC would need to establish more than the mere existence of internal access. They would need to show that, in substance, the annex was no longer suitable for use as a separate dwelling.

Practical Steps

If you are dealing with a similar HMRC challenge, gather evidence in a structured way:

• plans showing the layout of the main house and annex;
• sales particulars and survey reports;
• photographs from around the purchase date and after any works;
• evidence of separate facilities in the annex, such as kitchen, bathroom, sleeping and living space;
• contractor, architect or surveyor evidence explaining why any door or structural works were carried out;
• building regulation or fire safety material, if relevant;
• all HMRC correspondence, in date order;
• a short chronology explaining what existed at purchase and what changed later.

It is also important to identify the exact legal basis of HMRC’s challenge. Is HMRC saying:

• there were never two dwellings at the effective date of purchase; or
• there were two dwellings originally, but a later event reduced the number and triggered repayment?

Those are different arguments and should be answered differently.

If there is an appeal deadline, check it immediately and act within time. In SDLT disputes, the precise date of the assessment, amendment or closure decision matters.

Conclusion

For SDLT MDR purposes, the existence of a connecting door between a house and annex is not the end of the matter. The real test is whether the annex was, and remained, suitable for use as a single dwelling. Where a door was added or regularised for fire safety or structural reasons, and the annex still functioned independently, there may be a strong basis to resist HMRC’s attempt to treat the property as a single dwelling.

Legal References Used

• Finance Act 2003, section 58D
• Finance Act 2003, section 116(1)(a)
• Finance Act 2003, section 116(6)
• Finance Act 2003, Schedule 6B
• Finance Act 2003, Schedule 6B paragraph 7
• Amarjeet and Tajinder Mudan v The Commissioners for HMRC [2025] EWCA Civ 799

This page was last updated on 22 March 2026.

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