Accessory Dwelling Unit vs Annexe: Is there a difference?

When it comes to constructing an additional dwelling on your property; there are a lot of terms that refer to them floating around. From the traditional granny annexe to carriage houses and in-law suites; it easily becomes confusing as to how you should be referring to your build. An accessory dwelling unit (ADU) is one such example; but what exactly are they – and how do they differ from the typical annexe?

For the most part – the various terms and phrases used to describe secondary dwellings are interchangeable; and no-one is going to bat an eyelid regardless of what you call it. However, while many terms have little to no differences in definitions; some do have key differences that distinguish them from their counterparts. 

What is an Accessory Dwelling Unit?

The term ADU is a broad – and somewhat all-encompassing way of referring to secondary accommodation that occupies the same grounds as – or is attached to; a larger, single-family house. By rights – the owner of the ADU is also the owner of the main residence; meaning that it cannot be bought or sold separately – as a condominium or home on wheels might be.

As separate dwellings, ADUs must be self-sufficient for living independently. This means they must feature their own dedicated entrance – as well as the necessary kitchen, bathroom and living areas; for them to do so. What’s more, as they’re part of the same property; ADUs also make use of the same water, gas and electricity connections as the main house.

Within their criteria, ADUs can take on a multitude of different forms; from an apartment conversion above the garage; to a stand-alone backyard cottage and beyond. As such – while the term ‘accessory dwelling unit’ perfectly describes any additional living space; utilising other phrases might be more desirable – as it allows for more specificity to be given to the build.

How does an Accessory Dwelling Unit differ from an annexe?

If it sounds like there isn’t much of a difference between an accessory dwelling unit and an annexe; that’s because there isn’t really. While there are circumstances in which an ADU wouldn’t be classed as an annexe; these are incredibly rare – and the two terms can be used interchangeably for the most part.

As ‘accessory dwelling unit’ is a regulatory term; it’s one that tends to primarily be used by construction professionals and planners when referring to an annexe-like build. Besides this key difference, the term also applies more generally than phrases such as ‘granny flat’ or ‘granny annexe’. Indeed, whereas the former conveys the idea of a conversion above the garage; the latter would suggest more of a ground-level build – despite meaning the exact same thing.

Furthermore, there’s a key difference that distinguishes granny annexes from garden annexes; and despite how it might sound not – it’s not dependent on who will be living in it. Whereas the former can refer to an annexe that is either attached to, or detached from, the main property; the latter specifically refers to annexes situated in the garden – away from the main building. As such – while all garden annexes could be considered granny annexes; the same is not true vice versa. The term ADU negates all of this ambiguity, by simply referring to all secondary living as an additional dwelling – regardless of its form.

Other names for an Accessory Dwelling Unit

There are many more ways of referring to ADUs out there; each with their own subtle differences in definition. While it would typically be acceptable to use whichever term you prefer; it’s still important to familiarise yourself with the options available. 

1) Garden Pods and Garden Rooms:

While garden rooms tend to be just that – a singular room, albeit with all the mod-cons; garden pods can differ in their appearance and functionality. As a result, they often bear closer similarities to the traditional granny annexe. 

One important detail to note though – is whether the build is a self-contained property or not. While a granny annexe will always feature a devoted kitchen, bathroom and bedroom; garden pods could simply be an additional work space or living area – unless directly stated to be self-sufficient. Garden pods also mark a significant step-up from their more traditional counterparts – being more contemporary and modular in their design.

2) Tiny Houses:

Tiny houses are symbols – in the best way; of self-sufficiency, simpler living and virtuous modesty. Typically, they tend to be used as personal retreats and holiday-homes; as opposed to a form of full-time residence. Furthermore, tiny houses are usually constructed to be mobile; built on wheels rather than a more permanent foundation. 

So – while annexes and tiny homes both promote an independent lifestyle on a more-sustainable scale; there are some key differences that give the latter more specificity.

3) Carriage Houses:

A carriage house – also referred to as a ‘remise’ or coach house; is a type of outbuilding traditionally used to store a horse-drawn carriage. While the term refers to several different – and often overlapping, types of modern builds; the most common one is for an annexe-like property. 

Indeed, renovating a pre-built carriage house into a self-sustained secondary dwelling; is a popular option for many. It’s also worth noting that – while they can easily compare to modern garage conversions; carriage houses tend to detach from the main property.

4) In-Law and Au-Pair Suites:

An in-law – or ‘mother-in-law’ suite, is an additional living space; traditionally used to house the owners in-laws – as the name would imply. The term commonly refers to a small apartment-like space on the same property as – or even attached to; a singular family home. 

Similarly, an au-pair suite is another term that pays homage to the original intended use of an additional dwelling unit. Traditionally, the phrase ‘au-pair’ referred to a young foreign person taken in to help with housework or childcare; in exchange for a room, food and a little bit of money. However – unlike an employed nanny, they would typically be welcomed into the family as an equal member; and given the space as their own private quarters – even if it was only on a temporary basis. 

Both terms refer to a more traditional use for accessory dwelling units. While these phrases are used less frequently now-a-days; they can still be useful in defining the room as belonging to an old or historic building.

5) Ancillary Accommodation: 

An ancillary dwelling is a small, self-contained residence situated on the same lot as another property – much like granny annexes. Indeed, they too can be separate from the main house – or attached to it via an extension. Ancillary accommodation is always self-contained; meaning that other phrases – such as a garden room or pod, cannot be used to refer to them.

There are several complex laws defining what constitutes an ancillary building; but – in summation, they generally tend to require planning permission. In contrast, an incidental building – typically built using Permitted Development; would refer to smaller, non self-sufficient builds such as small extensions and outdoor storage.

6) Modular Buildings:

Modular building refers to the process of using pre-built, repeated sections – or modules; to form a completed unit. These modules tend to be constructed offsite – in a factory or workshop building; before being transported and assembled in its final destination. While this process often features in the construction of annexed properties; modular buildings do not have to be for self-sufficient living.

Indeed, unlike annexes, there are no limits as to their size; and modular buildings tend to be used for a wider variety of purposes – such as office buildings; alternative living facilities and retail units.

We use modular building to construct our annexes here at iHus; with the vast majority of the action taking place in our devoted factory in South Yorkshire. Not only is working this way incredibly efficient; it also means there are relatively few limits as to the style and footprint of our builds. What’s more – by constructing much of the annexe in our purpose-built premises; we reduce our greenhouse gas emissions by up to 30%. By eliminating much of the site traffic in the process; we also help keep our pollution on the roads down to a minimum.

Accessory Dwelling Unit? Just Keep Things Simple

With such a plethora of terms and phrases out there to refer to an accessory dwelling unit; it can easily get a little confusing as to what you should be calling yours. Let’s keep it simple – the most important name for it is home-sweet-home. So – whether your build is in the garden – or extended from the main house; just call it whatever you want. It’s your space to make your own, after all.

Leave it to iHus

We’re proud to be able to say we’ve delivered more than 300 bespoke annexes over the last 10 years. With all that experience under our belt; we’d like to think we know a thing or two about annexes; garden homes – or whatever term you’d prefer. We’re passionate about helping you create the perfect place to call home – get in touch with our friendly and devoted team today to find out how.

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If you would like to download our annexe brochure, please complete the form below. You can also request for a hard copy to be posted to your home address. Have any further questions, why not request one of our team get in touch?

If you would like to download our annexe brochure, please complete the form below. You can also request for a hard copy to be posted to your home address. Have any further questions, why not request one of our team get in touch?


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