What Are the Requirements to Build a Granny Flat?
Granny flats – also known as annexes or garden annexes in the UK – are becoming increasingly popular as multi-generational living becomes more common. Whether you’re looking to house elderly parents, provide accommodation for grown-up children, or simply create a self-contained space on your property, building a granny flat is a smart solution.
However, before you break ground, it’s vital to understand the legal, planning, and building requirements in the UK to ensure your project is fully compliant.
What Is a Granny Flat?
A granny flat is a self-contained living space built within the boundaries of a residential property. It may be attached to the main house, located within a converted garage or basement, or constructed as a separate outbuilding in the garden.
It typically includes:
- A bedroom
- A bathroom
- A kitchen or kitchenette
- A living space
Because it functions as an independent dwelling, it must comply with strict planning and building regulations.
1. Do You Need Planning Permission for a Granny Flat?
Yes, in most cases, building a granny flat requires planning permission. The reason is that a granny flat is more than just an outbuilding – it’s a habitable structure designed for permanent residence.
Exceptions:
Some granny flats can fall under Permitted Development Rights – but only if they are:
- Used by family members (not rented to others)
- Not marketed or sold separately
- Built within strict size and height restrictions
- Incidental to the main house and not used as a separate address
However, because the use is residential and often long-term, applying for planning permission is almost always required.
Tip: Always consult your local council before starting. Or speak with experts like Anx Space who can help you navigate the permissions process from start to finish.
2. Building Regulations Must Be Met
Even if your granny flat is permitted development, Building Regulations will apply. These regulations ensure the structure is:
- Safe
- Energy-efficient
- Structurally sound
- Equipped with proper drainage, plumbing, and electrical systems
Areas covered by Building Regulations:
- Foundations and structural integrity
- Thermal insulation
- Fire safety and escape routes
- Electrical wiring and sockets
- Plumbing and waste systems
- Ventilation
You’ll need to submit detailed plans and have your site inspected at key stages of the build.
3. Considerations for Services and Utilities
A granny flat should ideally be self-sufficient, so you’ll need to think about:
- Water supply and drainage
- Electricity (either connected to the main home or via a separate meter)
- Heating system (electric radiators, underfloor heating, or gas boilers)
Additionally, if you’re installing a full kitchen and bathroom, plumbing work will need to comply with regulations and may require separate permissions.
4. Council Tax Implications
If the granny flat is considered a separate dwelling, it may be subject to its own council tax band. However, there are discounts and exemptions available in certain cases – for example, if it is occupied by a dependent relative over 65 or someone with a disability.
You can contact your local council for details or let a professional like Anx Space advise you on the financial implications before you build.
5. Size, Location & Design Limitations
General Guidelines:
- The flat must be located within the residential boundary (curtilage) of your home
- Must not cover more than 50% of your garden area
- Height restrictions: usually up to 4 metres for a pitched roof or 3 metres for a flat roof
- It must be single-storey unless you apply for additional permissions
While these are general rules, each local planning authority may interpret them differently based on the neighbourhood, property size, and intended use.
6. Use Restrictions
Your granny flat should be used by:
- A family member
- A dependent relative
- Occasionally as guest accommodation
Using the space for short-term lets (e.g. Airbnb) or renting it out to unrelated tenants may require a change of use and different planning categorisation.
Common Questions
Q: Can I rent out my granny flat for extra income?
A: Not without planning permission. You’ll likely need a change of use, and it may affect your mortgage or insurance.
Q: Can I put a granny flat in a conservation area?
A: It’s possible, but more restricted. You’ll almost certainly need planning permission and must adhere to stricter design and material guidelines.
Q: How much does a granny flat cost?
A: Prices vary widely, but a typical build can range from £25,000 to £70,000+, depending on size, location, and specification.
Final Thoughts
A granny flat is a fantastic way to add flexibility, value, and multi-generational potential to your property. But with the added benefits come added responsibilities – particularly around planning, compliance, and usage.
By understanding the requirements ahead of time and working with the right professionals, you can make the process smooth and legally sound.
Need Help Building a Compliant Granny Flat?
The team at Anx Space can help you from initial consultation through to architectural design, planning applications, and construction. Whether you’re converting part of your house or building a separate annexe, they’ll ensure everything meets legal standards.
Book a Consultation Today with Anx Space
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