Granny Flat Cost Calculator UK
Estimate granny flat build costs — conversion, modular pod or new-build annexe.
Estimate Your Granny Flat Cost
How Much Does a Granny Flat Cost in the UK?
Granny flat costs vary greatly by type. A garage conversion costs £15,000–£35,000. A garden room annexe (modular pod) ranges from £40,000–£80,000. A purpose-built brick-and-block annexe costs £80,000–£150,000+. Timber frame annexes are slightly cheaper at £60,000–£120,000.
Key costs include: planning permission (£206 for householder application), building regulations fees (£800–£1,500), architects fees (£2,000–£5,000), utilities connection (£2,000–£10,000) and landscaping. Council Tax may apply as a separate banding after completion.
Frequently Asked Questions
How much does it cost to build a granny flat in the UK?
Costs range from £15,000 for a simple garage conversion to £150,000+ for a purpose-built brick annexe. Modular pods are mid-range at £40,000–£80,000.
Do I need planning permission for a granny flat?
Most annexes require householder planning permission (£206 fee). Some garden room pods under 15m² fall under permitted development but check with your local authority.
Can a granny flat have its own Council Tax band?
Yes — if the annexe is self-contained and used as a separate dwelling, the council may apply a separate Council Tax banding. A 50% discount may apply if a family member lives there.
How long does it take to build a granny flat?
Garage conversions take 4–8 weeks. Modular pods can be installed in 1–2 weeks. New-build annexes take 3–6 months including planning.
What is the cheapest type of granny flat?
Garage conversions are cheapest at £15,000–£35,000 provided the garage is attached and in good structural condition.
Do granny flats add value to a property?
Yes — a well-built annexe can add 20–30% to property value. Self-contained annexes are especially attractive to multigenerational buyers.
What utilities does a granny flat need?
Most annexes require electricity, water and drainage connections. A separate gas connection costs £1,000–£3,000 extra. Some opt for electric heating to avoid gas costs.
Can I rent out a granny flat?
Yes — rental income from a granny flat that shares the same plot is exempt from CGT on principal residence but is subject to income tax.
What is the Annexe Discount for Council Tax?
If a dependent relative aged 65+ or with a disability lives in the annexe, you may qualify for a 50% Council Tax discount on that unit.
What building regulations apply to a granny flat?
All new habitable spaces must comply with Part L (energy), Part B (fire), Part C (damp), Part F (ventilation) and Part M (accessibility) of the Building Regulations.
How do I insulate a granny flat?
Insulation standards (Part L) require walls to achieve 0.18W/m²K, roofs 0.15W/m²K and floors 0.13W/m²K. Triple-glazed windows are recommended.
Can a granny flat have its own address?
Yes — you can apply to the Royal Mail to register a separate address for a self-contained annexe. This is useful for insurance and correspondence.