Granny annexes sit in a grey area between garden rooms and self-contained dwellings. The planning rules are different, and getting them wrong can be costly. Here's what you need to know in 2026.
Granny Annexe vs Garden Room: What's the Difference?
A granny annexe is designed as living accommodation — typically with a bedroom, bathroom, and kitchenette. This distinction matters because:
- A garden room used as an office or gym is usually permitted development
- A self-contained dwelling (with sleeping, cooking, and washing facilities) often requires planning permission
- The key test is whether the annexe is "ancillary" to the main house or a separate dwelling
When You Don't Need Planning Permission
Your annexe may fall under permitted development if:
- It's genuinely ancillary to the main dwelling (used by family, not rented independently)
- It meets the size and height limits (single storey, max 2.5m eaves)
- It doesn't cover more than 50% of the garden
- It's not in front of the principal elevation
- Your property isn't listed or in a conservation area
When You Will Need Planning Permission
You'll almost certainly need a full application if:
- The annexe will be rented out or used as a separate household
- It exceeds permitted development size limits
- Your property has restricted permitted development rights
- The local authority considers it a new dwelling
Building Regulations
Regardless of planning permission, most granny annexes require building regulations approval. This covers:
- Structural safety
- Fire safety and means of escape
- Drainage and sanitation
- Electrical safety
- Energy efficiency
- Accessibility requirements
Council Tax Implications
An ancillary annexe used by a family member shouldn't attract separate council tax. But if the council classifies it as a separate dwelling, you may face:
- A separate council tax bill (though annexe discounts exist in some areas)
- Potential reclassification of your property
We always recommend getting a Lawful Development Certificate to confirm your annexe's status.
How We Help
At LouMil, we've built dozens of granny annexes across the Midlands. We handle the planning process, building control applications, and design your annexe to comply with all current regulations. Our team will advise on the best approach for your specific situation.
