Do you want to build a granny annexe or simply understand what it is? We cover all the basics of what is a granny annexe, the differences with a granny flat, planning requirements, costs and more. 

Table of contents: What Is a Granny Annexe? What Is the Difference Between a Granny Annexe and a Granny Flat? What Are the Planning Requirements for Granny Annexes? Caravan Act Lawful Development Certificate Householder Application How Large Can a Granny Annexe Be? How Much Does a Granny Annexe Cost? Granny Annexe Council Tax Exemption and Reduction Stamp Duty Relief for Granny Annexes

What Is a Granny Annexe?

In short, a granny annexe is a second building separate from the main house, usually located in the garden and built to house an elderly relative. 

These days granny annexes aren’t just for grannies or elderly family members though. They are often used to house younger family members as well – teens or young adults. Living with family close by means a greater sense of comfort and safety. It also makes financial sense. With housing prices outpacing salaries, it has become increasingly difficult for young people to get access to their own homes.

Granny annexes can be built without needing planning permission when they meet certain requirements. They are also exempt from council tax, when used for housing an elderly relative. And, when used by an adult child, for example, you can get a council tax reduction of up to 75%. We’ll cover the conditions in more detail further down. 

What Is the Difference Between a Granny Annexe and a Granny Flat?

Granny annexes and granny flats are used for the same purpose and the terms are often used interchangeably. We’ll look at the differences between the two and the most commonly used definitions for each term.

Granny flats are a part of the main building or house and have their own entrance, bathroom, living area, kitchen and bedroom. A granny flat can be a one or two-story addition that’s usually located at the back or side of the main house.

Granny annexes on the other hand are usually a separate building located further away from the main house. Originally meant to house elderly relatives, granny annexes and other types of garden rooms have become popular for many different uses: rental businesses, housing adult children, as garden offices and more.

Another thing to note is the size of the granny annexe. It’s smaller than the main residence and may need to meet certain requirements.

What Are the Planning Requirements for Granny Annexes?

Many factors come into play when it comes to building your granny annexe and whether you need to get a planning permission. Luckily, there are many garden buildings you can build without a planning permission. If you’re unsure whether you need one, it’s a good idea to read a granny annexe planning permission guide.

One of the most important things is to check if your granny annexe will be covered by the Caravan Sites Act.

Caravan Act

The Caravan Act allows for the construction of a mobile home without the requirement for planning permission. A mobile home means that the building must not be permanently fixed to the ground and needs to be able to be transported by a vehicle or trailer. 

Fortunately, even a building that is connected to utilities can be considered a mobile home under the Caravan Act. Another important aspect is that the building needs to be used by a family member and not be rented out or put up for sale. More on that later.¹

Lawful Development Certificate

You’ll also need to get a Lawful Development Certificate from your local authority. This is to confirm the use of the building and make sure that the granny annexe is considered an incidental building. A building that is established for incidental use means that it is considered to be a part or extension of the main house and not a separate dwelling.

This also means the annexe can not be sold separately in the future, since it’s considered a part of the main house.²

Don’t hesitate to consult a planning expert if you find yourself unsure about the rules and requirements in your area.

Householder Application

Should your annexe not be approved under the Caravan Act, you can apply for a Householder Application with your local LPA. You’ll be assigned a case officer who will handle your application. If you’ll be getting help through a planning service, their representative will be communicating with the case officer directly.³

How Large Can a Granny Annexe Be?

Local Planning Authority (LPA) regulations will be different between councils, but one general rule you can always be sure of is that the granny annexe needs to be smaller than the main house. LPA will also look at the combined footprint or size of the annexe and main house and then compare it to the land they are on. The more land is covered, the more challenging it is to get planning permission.

For the granny annexe to qualify as a mobile home under the Caravan Act it needs to meet the following size requirements:

• Less than 65.616 ft or 20 m in length (exclusive of any drawbar)
• Less than 22.309 ft or 6.8 m in width
• Less than 10.006 ft or 3.05 m in height (internal floor lowest point to internal ceiling highest point)

How Much Does a Granny Annexe Cost?

The price of a granny annexe will depend on many things like the size of the building and the price and quality of the chosen materials. Building a granny annexe can be quite costly, especially if you are using different contractors. Compliance with building regulations needs to be monitored at every step of the process. You’ll also need to get planning approval services and pay for materials and labour. You may be looking at about £80,000–£100,000 on average in costs.

One cost-effective solution is to get a prefabricated granny annexe. These buildings come in many different styles and sizes. You can choose a design you like, ask for modifications or even get a completely bespoke annexe. Prefabricated granny annexes are manufactured in a factory, then shipped in a flat-pack and can be swiftly assembled on-site. These kits cost about £10,000–£50,000. You’ll also have to factor in other costs, like installation, electrical work, etc.

Make sure to buy your prefabricated granny annexe from a trusted supplier like Summerhouse24. Look at reviews as well as how the buildings are constructed to get a good understanding of the quality. Don’t be afraid to ask for clarification to get a clear picture of what you’re buying. Most sellers will always be happy to answer all your questions.

Granny Annexe Council Tax Exemption and Reduction

When you’re housing an elderly relative or a family member in the annexe you either don’t need to pay the council tax on it or you can get a 50% or 75% reduction in most cases.

To get a full exemption you need to be housing a relative who is:

• Aged 65 or over
• Severely mentally impaired
• Substantially and permanently disabled

For a 50% reduction in council tax, the annexe needs to be used by yourself as the occupier of the main house or lived in by a relative. You may be able to get an additional 25% discount for a sole occupier, like an adult child that lives in the annexe.⁴

Stamp Duty Relief for Granny Annexes

Before June 1st, 2024, buyers got a reduction on stamp duty when buying a property with a granny annexe thanks to the Multiple Dwellings Relief.⁵ This relief has sadly now been abolished as many people were attempting to claim the relief when it wasn’t applicable.

Conclusion

Granny annexes are a popular addition to modern households because they are often the best solution to help our dependants or family members, increase our quality of life or even raise our property price. These days buyers often choose prefabricated granny annexes, because modern technology allows for cost-effective and sustainable manufacturing. 

Don’t hesitate to have a look at Summerhouse24’s One-Bedroom Cottage Holiday J or the popular 3-room Granny Annexe Oscar, both are great choices for a granny annexe. If you don’t see quite what you’re looking for among the hundreds of models available, you can easily request modifications or a bespoke project. With thousands of log cabins, annexes and garden rooms sold all across Europe and a 4.9-star rating on Trustpilot, you can be sure to receive the absolute best quality granny annexe from Summerhouse24.

Sources used:

1. https://www.legislation.gov.uk/ukpga/1968/52 
2. https://www.gov.uk/guidance/lawful-development-certificates
3. https://ecab.planningportal.co.uk/uploads/1app/guidance/guidance_note-householder.pdf 
4. https://www.warrington.gov.uk/discounts-annexes-and-granny-flats
5. https://www.gov.uk/guidance/stamp-duty-land-tax-relief-for-land-or-property-transactions