The Complete Guide to Granny Annexe Planning Permission in the UK

The Three Routes for Building a Granny Annexe

There are three main ways to build a granny annexe legally in the UK:

• Permitted Development: Building under existing rights without planning application
• Caravan Act Route: Classifying your annexe as a mobile home (often no planning needed)
• Full Planning Permission: Applying to your local authority for approval
• Each route has different requirements, benefits, and limitations

Route 1: Permitted Development Rights

Permitted development allows certain buildings without planning permission. For outbuildings (including annexes), the rules are:

• The building must not cover more than 50% of your garden (excluding the original house footprint)
• Maximum height of 4 metres for a dual-pitched roof, 3 metres otherwise
• If within 2 metres of a boundary, maximum height is 2.5 metres
• Cannot be forward of the principal elevation (front of your house)
• Must be single storey only
• Must be for purposes 'incidental' to the enjoyment of the dwelling
• No verandas, balconies, or raised platforms above 30cm

The 'Incidental Use' Problem

Here's where permitted development gets tricky for granny annexes. 'Incidental use' means the building should be subordinate to the main house. Think garden offices, gyms, or hobby rooms. The problem is that a self-contained living space with kitchen and bathroom goes beyond what most planning authorities consider 'incidental'. This means relying on permitted development alone for a full annexe is risky. If challenged, you could be required to remove the kitchen or even the entire building.

Route 2: The Caravan Act (Our Recommended Approach)

The Caravan Sites and Control of Development Act 1960 provides a better route for most annexes. Under this act, a structure is classified as a 'caravan' or 'mobile home' if:

• It's designed to be moved from place to place (in one or two sections)
• It doesn't exceed 20 metres in length and 6.8 metres in width
• It's constructed to BS 3632 standards for residential use
• It's used as residential accommodation ancillary to the main dwelling

Why the Caravan Act Route Works

The Caravan Act route is often the best option for granny annexes because:

• Placing a caravan in your garden for family use generally doesn't require planning permission
• The structure is zero-rated for VAT (saving you 20%)
• You avoid the 'incidental use' restrictions of permitted development
• The size limit (20m x 6.8m = 136 square metres) is generous for family living
• The build quality is identical to traditional construction
• The 'mobile' requirement means it can be relocated if needed

Route 3: Full Planning Permission

Some situations require a full planning application:

• You want a permanent, non-movable structure
• The annexe exceeds Caravan Act size limits
• Your property is in a conservation area, AONB, or National Park
• Your house is listed or in a listed building's curtilage
• The annexe will have its own postal address or council tax banding
• The annexe won't be occupied by a family member
• You want to sell or rent the annexe separately from the main house

The Planning Application Process

If you do need planning permission, here's what to expect:

• Pre-application advice: £100 to £300 for informal guidance from your council
• Application fee: Currently £258 for householder applications
• Required documents: Site plans, floor plans, elevations, design statement
• Consultation period: Neighbours are notified and can comment
• Decision timeline: Typically 8 weeks for householder applications
• Possible outcomes: Approval, approval with conditions, or refusal
• Appeal rights: You can appeal refusals to the Planning Inspectorate

What Planning Officers Look For

When assessing annexe applications, planning officers consider:

• Impact on neighbours: Overlooking, overshadowing, loss of light
• Design and appearance: Does it suit the character of the area?
• Size and scale: Is it proportionate to the main house and garden?
• Access and parking: Will it increase traffic or parking problems?
• Use: Is it genuinely ancillary to the main dwelling?
• Future use: Could it become a separate dwelling without control?

Conservation Areas and Special Designations

If your property has special status, the rules are stricter:

• Conservation areas: Permitted development rights are reduced. You'll likely need planning permission
• Areas of Outstanding Natural Beauty: Similar restrictions apply
• National Parks: Extra scrutiny on design and landscape impact
• Listed buildings: Any development in the curtilage needs listed building consent
• Article 4 directions: Some areas have removed permitted development rights entirely
• Always check with your local planning authority before assuming you don't need permission

The Importance of 'Ancillary' Use

Whatever route you take, the annexe must remain ancillary (secondary) to the main dwelling. This means:

• The occupant must be a family member or dependent
• The annexe should share some facilities or services with the main house
• There shouldn't be a separate postal address or council tax bill
• The annexe can't be sold or rented separately
• If questioned, you should be able to demonstrate the family connection
• Conditions may be attached requiring the annexe to remain ancillary

Building Regulations: A Separate Requirement

Planning permission and building regulations are different things. Even if you don't need planning permission, your annexe must comply with building standards:

• Caravan Act buildings comply with BS 3632 instead of building regulations
• Traditional builds require building regulations approval
• BS 3632 covers structure, insulation, electrics, plumbing, and fire safety
• Certification is provided on completion
• Our annexes are built to exceed minimum standards for comfort and efficiency

Common Planning Mistakes to Avoid

We've seen these mistakes cost people time and money:

• Assuming permitted development covers full annexes (it often doesn't)
• Not checking for conservation area or other designations
• Building first and seeking permission later (enforcement notices are serious)
• Forgetting to check restrictive covenants on your property deed
• Not considering neighbours' concerns before they object
• Choosing a design that doesn't suit the area's character

How We Handle Planning for You

At Grannexe, we manage the planning complexity on your behalf:

• Free site assessment to determine the best route for your property
• We design annexes to qualify under the Caravan Act where possible
• If planning permission is needed, we handle the entire application
• We liaise with planning officers and address any concerns
• We provide all required drawings and documentation
• Our experience means we know what works in different areas