An annexe can be a practical answer to many family situations in Cornwall: giving an older relative independence while staying close, creating space for grown-up children, or providing room for carers or visiting family. Whether it's a converted garage, an extension with its own front door or a separate building in the garden, an annexe needs careful planning. This guide explains how annexes typically work in Cornwall, from planning and ancillary use conditions to Building Regulations and the main build options.
What counts as an annexe?
In planning terms, an annexe is usually accommodation that is ancillary to the main house. That generally means it's used as part of the same household, shares the garden, access and often utilities, and isn't occupied as a separate, independent home. A typical annexe might have a bedroom, a shower room and a small kitchen or kitchenette, but how it's used matters as much as what's inside it.
This distinction is important. Accommodation used as a separate dwelling, for example let to someone outside the household or sold off, is normally treated as a new home and assessed very differently.
Annexe options
The main ways to create an annexe are:
Converting a garage or outbuilding. This often makes good use of an existing structure. Our guide to garage conversions in Cornwall covers the practical side.
An attached extension with its own entrance, linked internally to the main house. See our home extensions page.
A detached building in the garden, built in timber frame or masonry. Our garden offices page and our guide to garden rooms in Cornwall show some of the construction approaches.
Converting a barn or farm building on a rural property. Our guide to barn conversions in Cornwall explains the extra considerations.
A mobile or modular unit. In some circumstances these are treated differently in planning and Building Regulations terms, but the rules are specific, so it's worth taking advice before relying on this route.
Planning permission for an annexe
Permitted development rights allow some outbuildings without a planning application, but these rights are generally for uses incidental to the house, such as storage, a home office or a gym. Using an outbuilding as living accommodation with bedrooms, a kitchen and a bathroom usually falls outside them. As a result, most annexes in Cornwall will typically need planning permission from Cornwall Council.
When considering an application, the council will usually look at:
Whether the annexe is genuinely ancillary, including its size, its relationship to the main house and whether it shares access, parking and garden.
Design, scale and siting, particularly in conservation areas, near listed buildings or in the Cornwall National Landscape.
Impact on neighbours, including overlooking, light and noise.
Drainage, especially in rural areas without mains sewers.
Our guides to planning permission in Cornwall and building in an AONB or conservation area explain the process in more detail.
Ancillary use conditions
If permission is granted, it will often include a condition stating that the annexe must only be used for purposes ancillary to the main house, and that it can't be occupied, sold or let as a separate dwelling. Some permissions also restrict holiday letting. These conditions matter:
They stay with the property and affect future owners.
Breaching them can lead to enforcement action.
Changing how the annexe is used later, for example letting it independently or as a holiday let, would usually need a new planning application, and the outcome isn't guaranteed.
It's worth thinking about how you might want to use the annexe in five or ten years' time, not just today, and discussing this with your designer before applying.
Building Regulations
An annexe used as living accommodation will normally need Building Regulations approval, whether it's a conversion, an extension or a new building. Key areas include:
Fire safety, including smoke alarms, escape routes and, for detached buildings, separation from the boundary and fire service access.
Insulation and energy efficiency, which are particularly important when converting a garage or older outbuilding.
Ventilation and moisture control, especially in shower rooms and kitchens.
Drainage, with new connections to a sewer, septic tank or treatment plant.
Accessibility, which is worth thinking about early if the annexe is for an older or less mobile relative, even beyond the minimum requirements, for example level access, wider doorways and a walk-in shower.
Our guide to Building Regulations in Cornwall explains how approval works.
Council tax, CIL and other costs
There are a few financial points to check early:
Council tax. An annexe may be banded separately for council tax. Discounts or exemptions can apply in some circumstances, for example where it's occupied by a relative. Check the current rules with Cornwall Council.
Community Infrastructure Levy (CIL). Residential annexes may qualify for an exemption, but any claim usually needs to be made before work starts.
Services. Extending water, electricity, drainage and heating to a detached building can be a significant part of the cost, so allow for it from the start.
Costs vary widely depending on size, specification and whether you're converting or building new. A clear specification and a detailed estimate help you compare options. Our project management and QS services can help keep the budget on track.
Annexe FAQs
Can I rent out my annexe?
If your permission includes an ancillary use condition, letting the annexe independently or as a holiday let would usually need a new planning permission. Check your decision notice and take advice before changing how it's used.
Do I need planning permission to convert my garage into an annexe?
Converting a garage into a habitable room can sometimes be done without planning permission, but creating self-contained living accommodation often needs it, and conditions on earlier permissions may also apply. It's worth checking with Cornwall Council before you start.
How long does it take to build an annexe?
It depends on the type of annexe, planning, and the size of the build. A garage conversion is usually quicker than a new detached building, which may take several months once approvals are in place.
Talk to us about your annexe
If you're thinking about an annexe anywhere in Cornwall, we can help you weigh up the options and plan the build. Call us on 01208 816991 or contact us to talk through your plans.
