Granny Annexe Costs and Planning Rules
Build a self-contained annexe for family. Costs, planning permission rules, Building Regulations, council tax, and the best build options.
Multi-generational living is surging in the UK. Rising care costs, an ageing population, and adult children unable to afford their own homes are driving demand for self-contained annexes. Whether you call it a granny flat, granny annexe, or dependent relative unit, the concept is the same: an independent living space within your property for a family member.
Here's what it costs, what the rules are, and how to get it right.
Build Options
1. Modular / Prefabricated Annexe
Factory-built units delivered and assembled on a prepared base. The fastest and most predictable option.
- Cost: £40,000–£80,000 (1-bed, 30–50m²)
- Build time: 1–2 weeks on site (4–8 weeks total including base)
- Pros: Fixed price, fast, consistent quality, minimal site disruption
- Cons: Limited customisation, delivery access needed, may not match house style
- Best for: Quick installations, straightforward sites, budget certainty
2. Traditional Brick-Built Annexe
A conventional masonry construction built on site, matching the existing house.
- Cost: £60,000–£120,000 (1-bed, 30–50m²)
- Build time: 12–20 weeks on site
- Pros: Matches existing property, fully customisable, permanent structure
- Cons: More expensive, longer build, weather-dependent
- Best for: Permanent additions, larger budgets, design-sensitive sites
3. Garage / Outbuilding Conversion
Converting an existing detached garage or outbuilding into an annexe.
- Cost: £25,000–£60,000
- Build time: 6–12 weeks
- Pros: Uses existing structure, lower cost, often simpler planning
- Cons: Limited by existing footprint, may need significant structural work
- Best for: Properties with suitable existing outbuildings
4. Extension-Style Annexe
An attached annexe built as an extension to the main house but with its own entrance and self-contained facilities.
- Cost: £50,000–£100,000
- Build time: 12–18 weeks
- Pros: Shared wall reduces cost, direct connection to main house possible
- Cons: Uses garden space, affects the main house during construction
- Best for: Properties where a detached annexe isn't practical
Cost Breakdown: 1-Bed Annexe (40m²)
| Item | Modular | Traditional |
|---|---|---|
| Base/foundations | £3,000–£6,000 | £5,000–£10,000 |
| Structure | £25,000–£40,000 | £20,000–£40,000 |
| Roof | Included | £4,000–£8,000 |
| Windows and doors | Included | £3,000–£6,000 |
| Kitchen | £3,000–£6,000 | £3,000–£8,000 |
| Bathroom | £3,000–£6,000 | £3,000–£6,000 |
| Electrics | Included | £2,000–£4,000 |
| Plumbing and heating | Included | £3,000–£6,000 |
| Internal finishes | Included | £3,000–£6,000 |
| Drainage connection | £2,000–£5,000 | £2,000–£5,000 |
| Professional fees | £2,000–£4,000 | £4,000–£8,000 |
| Planning application | £258 | £258 |
| Building Regs | £500–£1,200 | £500–£1,200 |
| Total | £39,000–£73,500 | £53,000–£108,500 |
Regional adjustments apply - add 25–40% for London and the South East. Use our free calculator for a location-specific estimate.
Planning Permission
Why You Almost Always Need It
A self-contained annexe is legally a new dwelling, which requires planning permission. Key considerations the council assesses:
- Principle of development - is a new dwelling acceptable in this location?
- Impact on neighbours - overlooking, noise, parking pressure
- Design and character - does the annexe suit the area?
- Parking - most councils require at least one additional parking space
- Garden space - does enough usable outdoor space remain?
- Sustainability - drainage, flood risk, ecological impact
Dependent Relative Conditions
Many councils grant permission specifically for ancillary use - meaning the annexe must be occupied by someone connected to the main household. Common conditions include:
- The annexe cannot be sold, let, or occupied independently of the main house
- Occupation is restricted to family members or dependent relatives
- The annexe must remain ancillary (subordinate) to the main dwelling
- Some conditions fall away if the occupant's circumstances change
Check your council's policies. Submit a pre-application enquiry (£50–£250) through the Planning Portal before committing to design.
Lawful Development Certificate
If you believe the annexe qualifies as permitted development (rare for self-contained units, but possible for some non-habitable outbuildings), apply for a Lawful Development Certificate (£120) before building.
Building Regulations
Every habitable annexe must comply with Building Regulations:
| Requirement | Details |
|---|---|
| Structure (Part A) | Designed by a structural engineer |
| Fire safety (Part B) | Escape routes, smoke alarms, fire-rated construction |
| Drainage (Part H) | Connection to mains sewer or approved private system |
| Ventilation (Part F) | Openable windows, extract fans in kitchen and bathroom |
| Energy (Part L) | Full insulation, efficient heating, EPC assessment |
| Electrics (Part P) | Certified by a registered electrician |
| Access (Part M) | Level access, accessible bathroom if for a disabled occupant |
Part M (accessibility) is particularly relevant for annexes built for elderly or disabled relatives. Consider:
- Level threshold access (no steps)
- Wide doorways (minimum 800mm clear)
- Wet room or level-access shower
- Grab rails and support features
- Emergency call system
Council Tax
A self-contained annexe may be assessed for separate council tax by the Valuation Office Agency (VOA). However:
- Class S exemption: If the annexe is occupied by a dependent relative of someone in the main house, a 50% discount applies in England. Some councils apply full exemption.
- Empty annexe: May still attract council tax (with any applicable empty property discount)
- Attached vs detached: Both can be separately assessed if self-contained
Contact your council before building to understand the likely council tax position.
Services and Connections
Drainage
The annexe needs foul drainage connected to the mains sewer or an approved private treatment system. Options:
- Connect to existing house drainage - cheapest if the annexe is close (£1,000–£3,000)
- New connection to public sewer - if a separate run is needed (£2,000–£5,000)
- Private treatment plant - for rural properties not connected to mains sewerage (£5,000–£10,000)
Utilities
- Electricity: Can be supplied from the main house consumer unit via an armoured cable (£500–£1,500) or with its own supply (£1,000–£3,000)
- Water: Usually branched from the main house supply (£300–£800)
- Gas: Not always needed - electric heating or an air source heat pump avoids the need for a gas supply
- Internet: Run a Cat6 cable from the main house router (£100–£300) or install a mesh WiFi system
Heating
| Option | Cost | Running Cost | Notes |
|---|---|---|---|
| Electric panel heaters | £500–£1,000 | Higher | Cheapest install, most expensive to run |
| Air source heat pump (mini-split) | £2,500–£5,000 | Lowest | Heating + cooling, most efficient |
| Extend existing central heating | £1,500–£3,000 | Moderate | Only if boiler has spare capacity |
| Underfloor heating (electric) | £800–£1,500 | Moderate | Works well with tiled floors |
A mini-split heat pump is the best all-round choice for an annexe - efficient, provides heating and cooling, and avoids running gas or hot water pipes from the main house.
Annexe vs Other Options
| Option | Cost | Self-Contained? | Planning Needed? | Best For |
|---|---|---|---|---|
| Granny annexe | £40,000–£120,000 | Yes | Yes | Independent living for elderly parent |
| Garden room | £10,000–£30,000 | No (no sleeping) | Usually PD | Home office, studio, gym |
| Loft conversion | £30,000–£60,000 | No (part of house) | Usually PD | Extra bedroom within the house |
| Garage conversion | £10,000–£20,000 | No (part of house) | Usually PD | Extra room, not self-contained |
| Extension | £30,000–£80,000 | No (part of house) | Often PD | More space in the main house |
Finding a Specialist
For modular annexes, look for established manufacturers with a track record:
- Check they provide a structural warranty (typically 10 years)
- Ask whether they handle planning and Building Regulations on your behalf
- Visit a completed installation if possible
- Check reviews and ask for references
For traditional builds, use a builder experienced in small residential projects - find vetted contractors through the Federation of Master Builders or TrustMark.
Next Steps
- Check your plot - is there space for a detached annexe, or would an attached extension-style unit work better?
- Pre-application with your council - find out their policy on annexes before designing. Apply through the Planning Portal.
- Get 3 quotes - modular and traditional, to compare cost and timeline
- Plan for accessibility - if the annexe is for an elderly relative, design for their future needs
- Check council tax implications - contact the VOA and your council
- Get a cost estimate - use our free calculator for a starting figure
- Read about financing if borrowing for the project
Frequently Asked Questions
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