A Return to Multi-Generational Living
As the cost of housing, social care, and everyday life continues to rise, many households are turning to multi-generational living arrangements. The so-called “granny annex” has seen a resurgence — though these days it’s just as likely to house adult children, extended family, or even short-term Airbnb guests.
But as annexes become more common, they raise an increasingly important planning question: at what point does an annex stop being an annex and become a separate dwellinghouse?
The Planning Grey Area
As ever in planning this issue is a grey area (not to make planning sound dreary but everything these days seems to be grey, even the Green Belt!).
Time and again we see issues arising when it comes to planning and annexes.
Whether additional accommodation at a property is considered an annex or a separate dwellinghouse depends on the specific features and use of the accommodation on a case by case basis.
Although both annexes and separate dwellings typically fall under the same use class, the key difference is that:
- An annex is ancillary to an existing dwelling
- A separate dwellinghouse is an independent planning unit.
Crucially, many homeowners don’t realise that building a detached annex containing primary habitable accommodation always requires planning permission, even if it’s intended for family.
What Does an Annex Look Like?
One issue is that an ‘annex’ can take many forms. In some cases, it’s barely distinguishable from the main house, such as an attached garage conversion which has its own bedroom, bathroom and kitchenette, sometimes with its own access door but otherwise physically indistinguishable from the main house.
In other cases, they are detached outbuildings with varying levels of facilities and relationship with the main house. It’s the latter form which most often gets caught up in debate of whether a separate dwelling has been created.
Case Law and Key Precedents
Case law has established some circumstances which affect this assessment.
The most cited case is perhaps Uttlesford District Council v Secretary of State for the Environment & White [1991] 2 PLR 76. The LPA had argued that, by definition, the creation of a separate residential unit capable of independent occupation (regardless of how it was intended to be used) constituted a material change of use from a single dwelling to two independent dwellings.
The Court rejected this argument, confirming in line with the Inspector’s finding that whether a proposed annex creates a separate dwelling was to be decided on the facts of the case.
The Court confirmed that the Inspector was entitled to have made their decision based on the details of the case that the annex remained subordinate to the main house, and part of the same planning unit. The Inspector had decided this based on the joint access to the unit and the intention for it to be occupied by a family member requiring care with reliance on the main house for meals etc.
It also took no issue with the Inspector’s position that any future aspiration to occupy the annex as an independent residence would require planning permission.
Local Plan Policies and Their Limitations
Some LPAs have specific Local Plan policies which relate to annexes to seek to provide local clarity on the matter. However, this does not always help.
Case Study: Luton Appeal
In a recent case, we supported an appeal involving a converted detached garage in Luton, intended to accommodate our client’s elderly parents, who only live in the UK part-time.
Luton’s policy permits annexes for dependent relatives, provided that:
- A functional link exists (defined as an immediate family relationship);
- There is no boundary demarcation or sub-division of garden areas between the principal dwelling and the annexe; and
- Detached annexes must be fully ancillary in terms of size and facilities.
The policy then notes that if the annex would be self-contained so that it could function as an entirely separate dwelling, then the proposal would not be considered under this policy.
Despite meeting all criteria, the LPA argued the building was a separate dwellinghouse, largely due to its detached location and position on the front of the property.
Our appeal was allowed by the Inspector who concluded that the arrangement did comply with the Local Plan policy for annexes and the development was acceptable in all other regards.
In particular, the Inspector noted the personal circumstances of the family, the joint arrangement, and that from their site visit (during a time the parents were not using the annex) it was clearly being used as a home office.
Case Study: From Lawful Development Certificate to Airbnb Enforcement
In another case, an LPA mistakenly issued a Lawful Development Certificate (LDC) for an outbuilding shown to include a bedroom, shower room and utility kitchen area. Later, the building was used as an Airbnb, prompting the LPA to issue an enforcement notice, arguing it was an unlawful separate dwelling.
At appeal, the Inspector sided with the homeowner, stating:
“The outbuilding contains primary living accommodation [despite this] it could still be part and parcel of the primary dwellinghouse use if that use is physically and/or functionally connected to the use of the main house. The appellant has indicated that the accommodation is intended to be used in connection with the care and accommodation needs of her children, two of which are registered as disabled and live at home full time. In that instance, the use of outbuilding could be argued to be physically and functionally connected with the use of the house and a new planning unit would not be created.”
This shows the level of personal circumstance which can influence the outcome of an annex vs separate dwellinghouse argument, and perhaps in this case a little luck on the pragmatism of an Inspector.
Permitted Development and Conversions
Another source of confusion relates to permitted development (PD) rights.
Appeal decisions have determined that existing incidental outbuildings can be converted without it constituting a material change of use.
However, erecting a new detached outbuilding intended for primary residential use does require planning permission, (as it would not comply with the limitations of Class E).
Again, while the assessment depends on the specific circumstances, you could build a garden gym or office under Class E and then later start using it for sleeping accommodation – as long as there is no ambiguity over it being self-contained – this is not a change of use.
Categories of Residential Accommodation
To summarise, there are typically three broad ways to categorise additional accommodation at a property:
- Ancillary Accommodation
- Example: A garden room or garage conversion used as a bedroom or office.
- May include limited facilities, but remains dependent on the main house (e.g. do not typically have a full kitchen or bathroom).
- Annex
- Self-contained in terms of facilities but retains a functional link with the main house (e.g. it is only accessible through the garden of the main house, or it might be conditioned for family-only use on an ancillary basis).
- Tends not to be physically divided from the main house in any way (i.e. fenced off).
- Separate Dwellinghouse
- Fully self-contained and capable of independent occupation.
- Has its own access, facilities, and potentially private garden or amenity space.
- May be within the same curtilage as the main house but functions independently.
- May or may not be occupied by persons known to the main house.
Determining which category a particular setup falls into remains a case-by-case judgment – the nuances of the situation can easily tip the balance from one to another.
P.S. Annex or Annexe?
P.S yes, I did have an existential crisis about annex vs annexe when starting this article. So when is an annex actually an annexe?
Final Thoughts
The line between an annex and a separate dwellinghouse is not just a legal curiosity — it has real-world implications for planning permissions, enforcement, and family arrangements.
The bottom line – context matters. Whether a structure qualifies as an annex or not depends on the specific circumstances: its design, facilities, relationship to the main house, and the intent and nature of its use.
If in doubt — or planning to build — seek advice early, because what looks like a simple annex might be something quite different in planning terms.
