Residential annexes are Britain’s most overlooked housing opportunity: low-impact, family-friendly, and essential for scaling up new infill homes. Drawing on international success and UK examples, this piece sets out how a clear national framework and locally tailored pattern books could unlock a wave of small-scale homes.
The case for residential annexes
Residential annexes – also known as accessory dwelling units (ADUs), garden flats, granny flats, or ancillary accommodation – are small, self-contained homes built on the same plot as an existing property. They may take the form of a side extension, a converted outbuilding, a loft flat, or a timber-framed garden unit. What defines them is not their design, but their function: a small residence with its own kitchen, bathroom, and entrance.
Annexes are often built for family members, for example a grandparent needing care, or a child returning from university. These homes support care, preserve independence, and help families adapt to lifestyle changes.
Beyond the usual family benefits, residential annexes are also a useful tool for helping ease the housing crisis. Millions of rural and suburban homes sit on plots that are sufficiently large to host an annex, with no harmful visual impacts on the local area. When homeowners are given clear permission to build, this unused space can be unlocked — even in high-cost, low-density areas. This is precisely what happened in California.
How a simple rule change unlocked over 100,000 new homes
In 2016, California passed legislation permitting homeowners to build a small flat in their garage, garden, or attic. These residential annexes had long been possible in theory, but local authorities imposed complex rules, high fees, and discretionary processes that prevented them in practice.
That changed with the implementation of two state laws, SB 1069 and AB 2299, creating a right to build an Accessory Dwelling Unit (ADU) on nearly any single-family plot. This simplified designs, allowing automatic approvals in the case of council delays.
The result? A housing boom, with over 110,000 ADU permits issued – an increase of 15,000%. In fact, so many residential annexes were permitted that ADUs now account for one in every three new homes approved in Los Angeles.
Source: California YIMBY
What makes California’s story striking is that some of the sharpest increases in ADU permits came from some of the most unaffordable suburban areas, like Palo Alto, that had seen minimal housing delivery for decades. These areas had the space—but not the permission. Once the permits were brought in, homeowners were finally able to build, massively boosting numbers of homes built. The rise was intuitive: give people a clear, reliable route to build, and they will use it.
The Californian example shares similarities with practice in Sweden, the Netherlands, and Belgium, and is being taken further in Germany. These countries could serve as a model for delivering more homes in unaffordable areas in Britain too. They demonstrate how a small policy tweak can unlock thousands of new homes.
Rural councils leading the way
While Britain currently lacks a clear national framework for permitting residential annexes, some councils have taken steps to encourage them – especially in rural areas with slow housing delivery.
Twelve local authorities are currently leading the way. In the Peak District, residential annexes are actively supported in principle and in Cornwall they are defined as ‘acceptable’. Meanwhile, South Gloucestershire outlines a step-by-step checklist and West Suffolk supports compliant annexes. Find out more in the appendix.
While these local policies are promising, there is still no national right to build a residential annexe in England and therefore no sign of a California-style boom. But the use of pattern books, offering pre-approved designs tailored to local character, could feasibly shift the dial. By making annexes easier to approve, they offer a practical route to scaling up new infill homes.
Pattern books: a planning fix
One barrier to building residential annexes is local opposition, which often arises due to fears of low-quality outcomes. But pattern books offer a practical way to ensure high-quality designs that local communities can support . These are locally tailored design guides that set clear expectations for form, size, layout and materials. Crucially, they give certainty on what will be approved, reducing planning risk and unlocking delivery.
The Tai ar y Cyd programme in Wales – a collaboration between social landlords, government, and industry experts – shows how this could work in practice. It provides a pattern book of pre-designed homes, helping housing associations to deliver high-quality homes. While Tai ar y Cyd is in early stages, its potential is reflected in its recent award recognition.
The same pattern book principles could apply to residential annexes. By publishing a set of standard, high-quality designs, councils could unlock small-scale development without needing to write entirely new rules. Together, annexes and pattern books offer a win-win housing solution: a way to enable small-scale development that aligns with local character, meets growing need, and keeps families close. With the right tools, there’s no reason that Britain couldn’t deliver its own national annexe boom.
Appendix: local authorities permissive on residential annexes
- Amongst national parks the Peak District actively ‘supports residential annexes in principle’, while Dartmoor lets farms add a detached rural-worker annexe of up to 40 m² GIA .
- Across the South-West, Cornwall defines garden annexes as ‘often acceptable’ (usually kept below 50% of the host footprint), South Gloucestershire’s SPD outlines a step-by-step checklist, and North Somerset promises permission inside settlement limits.
- East Anglia goes further: East Suffolk’s Rural-Development SPD, Babergh-Mid Suffolk’s LP02 and West Suffolk’s LP29 all state that compliant annexes ‘will be supported/permitted’.
- Finally, Havant (Draft Policy 35), King’s Lynn & West Norfolk (DM7), East Herts (HOU 13) and North Northamptonshire (2021 SPD) promise the same green light—so long as the unit stays clearly subordinate, shares the main access/utilities and is tied by an occupancy condition.
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