Our Complete Guide to Building an Annexe in the Garden

What is a garden annexe, and what can it be used for?

A garden annexe is a permanent, self-contained, weatherproof building in the garden of an existing home, designed to be lived in year-round. Unlike a shed, summerhouse or garden office, it has its own kitchen, bathroom and living space and is built to residential standards.

Most are used as granny annexes — giving an older relative their independence while staying close to family — but they’re just as popular for adult children who can’t yet afford to move out, for guests, or as generous, private work-from-home space. An annexe is classed as ancillary to the main house, meaning it shares the property’s address and can’t be sold or let as a separate dwelling; it’s part of your home, and it adds lasting value to it.

Is your garden suitable for an annexe?

In most cases, yes. Your garden doesn’t need to be large, but a few things make a difference:

• Space — enough room for the annexe to sit without spoiling the look or usability of your garden. An annexe can’t take up more than 50% of the garden, or exceed the footprint of the main house.
• Layout — a corner or tucked-away spot usually works better than the middle of the lawn.
• Access — we need around a metre of access width, via a side gate or garage (not through the house), for the full duration of the build.
• Environmental factors — things like flood risk, Japanese knotweed, or unstable nearby trees are worth checking early.
• Utilities — access to water, electricity and drainage, though we handle the connections.

If you’re not sure whether your plot works, our specialists can tell you quickly. For the full picture, read can I build a granny annexe in my garden?

Planning permission and the Caravan Act

A granny annexe is a residential dwelling, so in most cases planning permission is required — and we handle the entire application on your behalf. We work with NAPC, the UK’s only dedicated annexe planning consultancy, submitting a Householder Application alongside a Certificate of Lawfulness under the Caravan Act. This dual approach gives you the best possible chance of approval, and we currently have a 94% success rate.

Be wary of anyone claiming the Caravan Act lets you skip planning entirely — it only applies to structures that meet specific criteria, and most garden annexes are still treated as permanent dwellings. We explain exactly how this works in can I build a granny annexe in my garden?

Designing your annexe

Your annexe is built around the people who’ll use it. Our Classic range is fully customisable — layouts, one or two bedrooms, kitchens, bathrooms, flooring, cladding, windows and roof style (a flat EPDM roof as standard, or a pitched roof for an additional cost). Our 3632 range offers fixed, contemporary designs selected for functionality and a modern feel.

Accessibility is often a priority, so features like wide doorways, step-free access and wet rooms are all available, and every annexe is insulated and built to be energy-efficient for year-round living. Solar panel packages are available across the range, and come as standard on our contemporary Pent Hus models.

For the detailed design questions — sizes, layouts, customisation and accessibility — see our granny annexe design, layout & build FAQs, or browse our range of annexe designs.

How an annexe is built, and how long it takes

We build on ground screw pile foundations — an eco-friendly, low-disruption alternative to traditional concrete footings. The timber structure is manufactured in cassette form at our factory in South Yorkshire, then delivered to your site and constructed in place. We connect water, electricity and drainage (a septic tank is no problem if you’re not on mains sewerage); we don’t use gas.

On timescales: once your planning application is validated, which can take up to 4 weeks, the planning process itself takes around 8 weeks. The build then takes a further 8 to 12 weeks, depending on the size and specification you choose.

What does a garden annexe cost?

An iHus annexe is a single, fixed, fully turnkey price. That covers the whole project — in-depth consultation, site survey, architectural design and CGI drawings, planning support, groundworks and foundations, the build, utility connections, and interior finishes — rather than leaving essentials as costly extras. Where your annexe sits in the range depends on its size, layout, materials and finishes.

For current prices and exactly what’s included, see how much a self-contained granny annexe costs. It’s also worth knowing what a cheap quote often leaves out, so you’re comparing like with like.

VAT, council tax and running costs

Annexes that comply with the Caravan Act are VAT exempt. Most councils apply a 50% council tax discount, and where a dependent relative lives in the annexe — someone over 65, substantially or permanently disabled, or severely mentally impaired — it may be fully exempt. Day-to-day running costs are low, as an annexe is smaller and more energy-efficient than a main home.

For the full detail on payments, VAT, mortgages, insurance and guarantees, see our granny annexe costs, finance & tax FAQs, and council tax specifically in our guide to granny annexe council tax rules.

Is a garden annexe worth it?

For most families, yes. An annexe adds lasting value to your property, costs far less than years of residential care, and keeps a loved one independent but close. It’s an investment in both your home and your family rather than simply an expense. We look at the numbers, including the comparison with care costs, in is a granny annexe worth it?

The iHus process, from consultation to moving in

It starts with a free, no-obligation consultation, by video or in person. Your consultant listens to what you need, then produces a detailed quote along with drawings and CGI of your annexe inside and out. Next comes a full site survey, where we mark out the location, take measurements, and bring samples of everything from cladding to kitchen units. From there we handle planning, the factory build, installation and handover — and we check back in at six months to see how you’re getting on. Every annexe comes with a 10-year guarantee.

Prefer to see one first? Our Open Hus network lets you visit a lived-in annexe and speak to real customers. And for exactly what to expect from your appointment, read your iHus consultation: everything you need to know.

Why choose iHus?

In 20 years we’ve built annexes for over 500 families across England and Wales, working with more than 250 local planning authorities and a choice of 12-plus designs. With a 94% planning success rate, a genuinely all-inclusive turnkey service, a 10-year insurance-backed guarantee and aftercare that continues long after handover, you’re in safe hands from the first phone call to the day you move in.

Ready to start?

The best first step is a free, no-obligation consultation. Speak to our team and we’ll talk you through what’s possible in your garden.