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EXTENSIONS & RENOVATIONSGranny Annexe Costs and PlanningRules
Extensions & Renovations8 min read1 April 2026

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

  1. Check your plot - is there space for a detached annexe, or would an attached extension-style unit work better?
  2. Pre-application with your council - find out their policy on annexes before designing. Apply through the Planning Portal.
  3. Get 3 quotes - modular and traditional, to compare cost and timeline
  4. Plan for accessibility - if the annexe is for an elderly relative, design for their future needs
  5. Check council tax implications - contact the VOA and your council
  6. Get a cost estimate - use our free calculator for a starting figure
  7. Read about financing if borrowing for the project

Frequently Asked Questions

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