A granny annexe — a self-contained living unit within or adjacent to a family home — has become one of the most sought-after home improvements in the UK as multigenerational living grows in popularity. The cost varies enormously depending on whether the annexe is attached to the main house, converted from an existing structure, or built as a standalone garden building. In 2026, costs range from £25,000 for a basic attached conversion up to £160,000 or more for a detached timber-frame lodge with full services.
The planning and legal picture is more complicated than a standard extension — get this right before committing to any design.
Types of Annexe and Typical Costs
| Annexe Type | Description | Typical Cost 2026 |
|---|---|---|
| Attached annexe (integral conversion) | Converts part of the main house — side return, garage, or ground floor | £25,000–£65,000 |
| Above-garage annexe | New first floor above an existing detached garage | £40,000–£80,000 |
| Garden annexe (modular/timber frame) | Prefabricated unit in the garden | £60,000–£130,000 |
| Detached purpose-built annexe (brick/block) | New build in the garden | £90,000–£160,000+ |
Costs include structure, insulation, services (water, drainage, electrics), kitchen, bathroom, and a basic fit-out. They do not include professional fees (typically 10–15% of build cost) or the cost of laying a new utilities spur from the street.
Planning Permission
Planning permission is almost always required for a self-contained annexe — whether attached or detached — because the unit must be “ancillary” to the main dwelling and cannot be occupied as a separate household in its own right under most residential planning consents.
When you may not need planning permission:
- An integral conversion of existing floor space (e.g., converting a large ground-floor room to an annexe with a wet room) may fall under Permitted Development if no external alterations are needed.
- A small outbuilding in the garden may be Permitted Development under Class E rules if it is not self-contained or used as sleeping accommodation — but this is rarely sufficient for a true annexe.
What to watch for with planning applications:
- Local authorities often impose a condition that the annexe cannot be sold separately from the main house or let to an unrelated person. This is called a “no separate disposal” or “no separate letting” condition and is standard practice. It is meant to prevent the annexe becoming a separate dwelling.
- Planning policy in many areas treats a self-contained annexe as a separate dwelling (Use Class C3) if it has its own entrance, full kitchen, and bathroom. This can trigger the full planning process including design, access, and amenity assessments.
- In Green Belt, AONB, or National Park locations, the bar is higher; even ancillary annexes can be refused.
Always consult a planning consultant or submit a pre-application enquiry with the local planning authority before commissioning drawings. This costs £50–£250 and provides written guidance from a planning officer.
Building Regulations
Building Regulations apply to all annexe work — both the structural elements and the services. The key approved documents:
- Part A (Structure): foundations, walls, and roof assessed for the new or converted structure.
- Part B (Fire safety): means of escape, fire separation between annexe and main house if attached, smoke alarms.
- Part F (Ventilation): habitable rooms require background and rapid ventilation; kitchens and bathrooms need extract fans.
- Part G (Sanitation): new drainage connections require Building Regulations approval.
- Part L (Energy efficiency): U-values for new walls, floors, and roofs must meet 2022 standards (walls ≤ 0.28 W/m²K, floor ≤ 0.22, roof ≤ 0.16).
- Part P (Electrics): notifiable electrical work must be certified by a registered electrician.
For a detached garden annexe, the structure effectively requires the same scrutiny as a new dwelling — the Building Control process is therefore more involved than a single room conversion.
Council Tax
This is the most commonly overlooked aspect of annexe planning. A self-contained annexe — defined as having its own entrance, kitchen, and bathroom — can attract a separate Council Tax band, effectively making it a second property in the eyes of the local authority.
There are two important reliefs:
- Annexe occupied by a relative: in England, a self-contained annexe that is the main home of a dependent relative (elderly, disabled, or a student) is exempt from Council Tax under the Council Tax (Exempt Dwellings) Order 1992.
- 50% discount: if the annexe does not qualify for full exemption, a 50% Council Tax discount applies in England when the annexe is occupied by someone related to the main house occupier.
Check the current position with your local council before completing the annexe, as the relief must be applied for actively and is reviewed periodically.
Can You Let It Out?
In most cases, the planning condition attached to an annexe consent will restrict lettings to family members or persons ancillary to the main household. Renting the annexe commercially on a separate assured shorthold tenancy is likely to breach planning conditions, which could result in enforcement action and a requirement to demolish or cease use.
If you want flexibility to let commercially, engage a planning consultant from the outset to explore whether a separate residential dwelling consent is achievable on the plot. This changes the economics and the planning process significantly.
Garden Annexe: Modular vs Traditional Build
Modular and timber-frame garden annexes have become a popular route because they arrive largely prefabricated, reducing on-site time to 4–12 weeks. Traditional brick-and-block takes 4–6 months.
| Factor | Modular/Timber Frame | Brick and Block |
|---|---|---|
| Build time on site | 4–12 weeks | 4–6 months |
| Cost | £60,000–£130,000 | £90,000–£160,000+ |
| Longevity | 30–60+ years (depends on spec) | 60–100+ years |
| Planning | Same process applies | Same process applies |
| Flexibility of design | Limited by supplier range | Fully bespoke |
| Thermal performance | Excellent if well-specified | Excellent |
Modular units vary in quality — ask for U-value certificates and enquire about the air-tightness figure (the SAP calculation) before committing. A poorly specified timber panel can be cold and damp; a well-specified one outperforms brick.
Key Questions to Ask Before Starting
- Has your planning consultant confirmed the scheme has a realistic prospect of consent?
- Does the local authority’s standard condition allow the intended occupant (e.g., a non-relative carer)?
- Will a separate utilities spur be needed, and what is the cost of connection from the street?
- Has a structural engineer reviewed the ground conditions for a new foundation?
- Have you budgeted for professional fees (architect/planning consultant, structural engineer, Building Control)?
Approximate Cost of Professional Fees
| Professional | Typical Fee |
|---|---|
| Architect / designer (drawings + planning) | £3,000–£8,000 |
| Planning application fee (England, householder) | £258 |
| Structural engineer | £800–£2,000 |
| Building Control (private Approved Inspector) | £800–£2,500 |
| Party wall surveyor (if applicable) | £700–£2,000 |
A granny annexe done well adds significant value and functionality to a family home. Done poorly — without planning permission, or with a flawed connection to mains drainage — it can create legal and financial headaches that far outweigh the benefit.
