When you ask 'how much does a granny annexe cost?', the honest answer is 'it depends'. Not because we're being evasive, but because every project is different. Two annexes of the same size can vary by £20,000 or more depending on specification, site conditions, and location. Understanding these cost factors helps you budget realistically and make informed decisions about where to spend and where to save.
Factor 1: Size (The Biggest Variable)
Floor area is the primary cost driver. More space means more materials, more labour, and higher cost:
- Studio (30 to 40 sqm): £90,000 to £105,000
- One bedroom (45 to 55 sqm): £110,000 to £130,000
- Two bedroom (55 to 70 sqm): £130,000 to £155,000
- Larger or custom designs: £155,000 to £200,000+
- Roughly speaking, expect £2,000 to £2,500 per square metre
- Bigger isn't always better; right-size for your needs
Factor 2: Specification and Finishes
The level of finish dramatically affects price. Options range from standard to premium:
- Standard kitchen: Quality units, laminate worktops, integrated appliances
- Premium kitchen: Shaker doors, granite or quartz worktops, premium appliances
- Standard bathroom: White suite, electric shower, vinyl flooring
- Premium bathroom: Wetroom, rainfall shower, tiled throughout
- Flooring: Vinyl or laminate (standard) versus engineered wood or tile (premium)
- Specification upgrades can add £5,000 to £20,000 to base price
Factor 3: External Cladding
What the annexe looks like from outside affects cost and planning:
- Composite weatherboard: Most affordable, low maintenance, modern look
- Timber cladding: Mid-range, natural appearance, periodic treatment needed
- Green oak: Premium, beautiful natural aging, higher cost
- Brick slips: Premium, blends with brick houses, planning-friendly
- Natural stone: Highest cost, for conservation areas and high-end projects
- Cladding choice can vary costs by £3,000 to £15,000
Factor 4: Roof Style
The roof design affects both appearance and cost:
- Flat roof: Most affordable, contemporary look, garden-room aesthetic
- Mono-pitch: Slightly higher, interesting angle, good ceiling heights
- Dual pitch (gabled): Traditional look, higher cost, good for planning
- Roof covering: Felt (economical) versus tiles (premium, traditional)
- Vaulted ceilings: Open interior adds cost but creates dramatic space
- Roof style can vary costs by £2,000 to £8,000
Factor 5: Heating System
How the annexe is heated affects both build cost and running costs:
- Electric panel heaters: Lowest install cost, higher running costs
- Air source heat pump: Higher install cost (£3,000 to £5,000 more), lower running costs
- Underfloor heating: Premium comfort, add £2,000 to £4,000
- For long-term value, heat pumps often make sense
- Consider occupant comfort and ease of use
Factor 6: Site Access
How easy it is to get the annexe to your garden affects cost significantly:
- Good access (3m+ width, clear route): Included in standard pricing
- Restricted access (narrow passages, gates): Add £2,000 to £5,000
- Very difficult access (crane over house): Add £5,000 to £10,000
- Road closure permits if needed: £500 to £1,500
- Good access saves money; assess this early
Factor 7: Distance from Utilities
Your annexe needs connection to electric, water, and drainage:
- Close to services (under 15m): Included in standard pricing
- Longer runs: Add approximately £100 to £150 per metre
- 30m service run: Add approximately £2,000 to £3,000
- Complex drainage solutions: Can add £2,000 to £5,000
- Position annexe to minimise service distances where possible
Factor 8: Ground Conditions
What's underneath your garden affects base and groundwork costs:
- Level, stable ground: Standard base included
- Sloping site: Stepped base or retaining may add £2,000 to £8,000
- Poor drainage: Remediation add £1,500 to £4,000
- High water table: Specialist solutions may be needed
- Existing structures to remove: Depends on size and type
- Ground conditions are assessed during site survey
Factor 9: Planning Route
How you gain permission affects timeline and cost:
- Caravan Act route: Zero VAT (20% saving), often no planning fees
- Permitted development: No application fee, but may need lawful development certificate (£130)
- Full planning permission: Application fee (£258), possibly architect fees
- Conservation area: May need heritage consultant, specialist materials
- The Caravan Act route saves the most money for most families
Factor 10: Location in the UK
Where you live affects prices to some degree:
- London and South East: Typically 5 to 10% higher than national average
- Rest of South and Midlands: Around average
- North and Scotland: Potentially slightly lower
- Remote locations: Delivery costs may be higher
- Labour costs and site access typically explain regional variations
What's Included in Our Prices
Our quotes include everything needed for a complete, liveable annexe:
- Complete structure with full insulation
- Full electrical installation with certificate
- Complete plumbing for kitchen and bathroom
- Fitted kitchen with appliances
- Complete bathroom with shower, toilet, basin
- Flooring throughout
- Internal doors and finishing
- External cladding and windows
- Heating system
- Standard service connections
- Delivery and installation
- No hidden extras for a standard project
What Might Be Extra
Some items may be additional depending on your site:
- Difficult access solutions
- Long service runs
- Significant groundwork for slopes or poor conditions
- Planning application fees if needed
- Specification upgrades beyond standard
- External works (decking, paths, landscaping)
- We make extras clear in our detailed quotation
Where to Save and Where to Spend
Our recommendations for budget allocation:
- Don't save on insulation: Comfort and running costs depend on it
- Don't save on bathroom: Used daily, affects quality of life
- Can save on external cladding: Composite looks great at lower cost
- Can save on kitchen upgrades: Easier to upgrade appliances later
- Consider heating carefully: Higher upfront for heat pump saves long-term
- Prioritise what the occupant will use and enjoy most
Conclusion
Granny annexe costs vary because projects vary. Understanding the factors that drive price, from size and specification to site conditions and access, helps you plan realistically and make informed choices about where to invest and where to economise. The best way to get an accurate figure for your project is a detailed site survey where we assess your specific garden, discuss your requirements, and provide a comprehensive quotation. Our surveys are free and without obligation. Call us on 01689 818400 to arrange yours.
Frequently Asked Questions
Why do different companies quote such different prices?
Huge variations in quotes usually mean different things are included. Check whether quotes include VAT (ours are zero-rated under Caravan Act), service connections, kitchen appliances, and site preparation. Like-for-like comparison often shows prices are closer than they appear.
Can I save money by doing some work myself?
We provide turnkey solutions because partial involvement creates warranty and coordination complications. However, you could reduce costs by handling landscaping, external paths, or some internal decorating after handover.
Is it cheaper to build in winter?
Lead times may be shorter in winter (quieter period), but build costs are similar year-round. Factory construction means weather has minimal impact on our process. Some families prefer to commission in winter for spring completion.
Do prices increase each year?
Building costs have been rising with inflation and material price pressures. A quote today is typically valid for 60 to 90 days. If you're planning 6 to 12 months ahead, budget for potential increases of 3 to 5%.
What's the cheapest annexe you can build?
Our most affordable option is a studio annexe at around £90,000. We wouldn't recommend going much below this for genuine permanent residence; cheaper options often compromise on insulation, finishes, or compliance in ways that affect comfort and durability.