Granny Annexe London 2026: Planning, Costs & What's Allowed
A granny annexe is one of the most useful things you can build on a London property — but the planning rules are more complicated than most homeowners realise. Here is what actually determines whether you need permission, what it costs, and what the financial implications are.
Quick Answer
A self-contained granny annexe always needs planning permission regardless of size or whether it is attached or detached. An incidental outbuilding (no self-contained kitchen, bathroom, or sleeping accommodation as a primary function) may qualify as permitted development under Class E. Costs inc VAT: garage conversion £25k–£50k, attached extension annexe £60k–£120k, detached outbuilding annexe £50k–£120k.
£25k–£50k
Garage conversion
£60k–£120k
Extension annexe
£50k–£120k
Detached outbuilding
Check your specific property constraints
Free Property CheckWhat Is a Granny Annexe?
A granny annexe is additional living accommodation within the curtilage of an existing house, intended for a family member — typically an elderly parent, an adult child, or a dependent relative — to live alongside the main household while maintaining some degree of independence.
In practice, annexes in London take three forms. Which form you choose affects the planning route, cost, and what is legally permissible without a planning application.
Garage conversion annexe
The most affordable route. An integral or detached garage is converted into living space and connected — physically or via a shared entrance — to the main house. Works best where the garage has reasonable ceiling height (minimum 2.4m) and does not need to be excavated to meet floor level requirements. Typical cost: £25k–£50k inc VAT.
Attached extension annexe
A new side or rear extension built onto the house, fitted out as self-contained or semi-independent accommodation. Often the most practical in terms of managing shared services (heating, utilities) and provides the best level of comfort. Typical cost: £60k–£120k inc VAT depending on size.
Detached outbuilding annexe
A new building in the garden, separate from the main house. Offers the greatest privacy and independence. Planning implications depend heavily on whether the accommodation is self-contained. Typical cost: £50k–£120k inc VAT depending on size, specification, and whether connections (drainage, electricity, heating) need to be run across the garden.
The Planning Question: Self-Contained or Incidental?
The single most important planning question for any annexe is whether it will be self-contained or incidental. This distinction determines whether you need planning permission, and whether permitted development rights apply.
What makes an annexe self-contained
An annexe is self-contained when it provides everything needed for independent living. Planners look for:
- Its own kitchen (or kitchenette) capable of full meal preparation
- Its own bathroom or shower room
- Sleeping accommodation (bedroom)
- Its own external entrance (does not require access through the main house)
- Separate utility metering or the ability to be separately let or sold
Key rule: A self-contained annexe is a separate dwelling in planning law. It does not matter that it sits within your garden or that a family member will live in it. Self-contained always means planning permission is required — no exceptions.
What incidental means
An outbuilding is "incidental to the enjoyment of the dwellinghouse" when it supports the use of the main house rather than enabling independent living. Classic examples: a home office, gym, hobby room, garden store, or games room. The GOV.UK technical guidance for householder permitted development makes clear that the incidental use test cannot cover self-contained accommodation or any outbuilding used for primary living accommodation such as a bedroom, bathroom, or kitchen.
This is where many homeowners get into difficulty. They assume that because they are not renting it out and a family member will be living there, it does not count as a separate dwelling. Planning law does not work that way. The question is what the accommodation is capable of, not who will occupy it.
Planning Permission for a Self-Contained Annexe
If your annexe will be self-contained, you need full planning permission. This is a standard householder application — the same process as for an extension — but councils assess it differently because the use is residential rather than ancillary.
What councils assess
The annexe should be clearly secondary to the main house in terms of size, layout, and relationship. A detached building that is larger than the house itself is unlikely to be approved as an annexe and may be considered a separate dwelling house on its own plot.
Many councils favour annexes with a physical connection to the main house (shared wall, connecting door) rather than fully detached structures, as this reinforces the subordinate and ancillary character.
As with any extension, bulk, massing, materials, and impact on neighbours are assessed. A detached outbuilding in the rear garden must meet the same design quality standards as a rear extension.
Councils routinely attach occupancy conditions to annexe planning consents. The most common condition restricts use to "ancillary residential accommodation for members of the household of the main dwelling only" and prevents the annexe being sold or let separately. This condition runs with the land permanently — it binds all future owners.
Planning applications for annexes in London boroughs are assessed under both national planning policy and local policies. Most inner London boroughs have policies specifically addressing self-contained annexes and garden structures. Camden, Hackney, Islington, and Lambeth in particular apply these policies carefully in areas where loss of family-sized homes or garden land is a planning concern.
Application fee: £528 (householder application). Decision: 8–10 weeks in most cases.
Check your annexe planning status
Tell us your property address and annexe type. Our AI checks conservation area status, Article 4 directions, and gives you a realistic assessment of which planning route applies.
Permitted Development for Outbuilding Annexes
A detached outbuilding in the rear or side garden of a house can be built under permitted development (Class E of Schedule 2, Part 1 of the GPDO 2015) without any application — but only if it is incidental to the enjoyment of the dwellinghouse.
Class E conditions
To qualify under Class E, the outbuilding must meet all of the following:
- Not forward of the principal elevation (not in front of the house)
- Not within the curtilage of a listed building
- Maximum height 4m (dual-pitched or hipped roof) or 3m otherwise
- Eaves no higher than 2.5m if within 2m of the curtilage boundary
- The total ground area covered by outbuildings and other structures (excluding the original house) must not exceed 50% of the total curtilage
- Not a separate dwelling — use must be incidental to the main house
The sleeping accommodation question
The GOV.UK technical guidance for householder permitted development is explicit: Class E does not cover "the use of an outbuilding for primary living accommodation such as a bedroom, bathroom, or kitchen."
In practice, a garden room with a sofa bed used occasionally as a guest room is treated differently from a structure permanently laid out as a bedroom with ensuite and kitchenette. The latter would be considered primary living accommodation regardless of whether it technically has a separate front door.
If you want certainty, apply for a Lawful Development Certificate before building. This formally confirms to your local planning authority that the proposed use is lawful — and protects you if the use is later questioned.
Conservation areas and Article 4 directions
In a conservation area, outbuildings in the rear garden of a house can still be built under Class E — the conservation area restriction primarily limits what can be done to the front of the property. However, if the outbuilding is in the side garden and visible from the road, or the council has applied an Article 4 direction removing outbuilding permitted development rights, planning permission will be needed.
Most inner London boroughs have extensive Article 4 directions. Check the planning constraints at your address before assuming permitted development applies.
Not sure whether your annexe needs planning permission? Our AI checks your address, property type, and conservation area status instantly.
Check now — it's free →Granny Annexe Costs in London 2026
All figures below include VAT at 20%. Construction cost is the build contract only. All-in adds architect, structural engineer, building regulations, and a 10% contingency.
| Type | Construction | All-in |
|---|---|---|
| Garage conversion (basic) | £18k–£35k | £25k–£50k |
| Side extension annexe (20–30m²) | £50k–£90k | £60k–£108k |
| Rear extension annexe (20–30m²) | £55k–£100k | £66k–£120k |
| Detached outbuilding (20–35m²) | £42k–£100k | £50k–£120k |
What adds to cost for an annexe specifically
A detached outbuilding is often cheaper on the build itself than an attached extension of the same size, but the cost of running all services — mains electricity, water supply, foul drainage, heating — across the garden can add £4,000–£12,000 that an attached annexe avoids entirely. Get quotes for both before assuming a detached structure is cheaper overall.
Building Regulations
All habitable annexes — whether attached or detached, with or without planning permission — require building regulations approval. The standards that apply are higher than for a non-habitable outbuilding.
Habitable space standards
A room used for sleeping must achieve minimum ceiling height (2.1m minimum, 2.3m preferred), adequate floor area, ventilation, and natural light. Kitchens and bathrooms require mechanical ventilation and specific drainage arrangements. Building control will assess the annexe as habitable space, not as a garden outbuilding.
Insulation (2026 standards)
- Walls: U-value 0.18 W/m²K or better
- Roof: U-value 0.15 W/m²K or better
- Floor: U-value 0.13 W/m²K or better
- Glazing: U-value 1.4 W/m²K or better
A detached outbuilding used as a garden room has no insulation requirement under building regulations. The moment it becomes habitable accommodation these standards apply in full.
Part E sound (attached annexes)
Where an annexe shares a wall, floor, or ceiling with the main dwelling, Part E of building regulations (resistance to sound) applies. The shared element must achieve defined performance standards for airborne sound (DnT,w) and impact sound (L'nT,w). This typically requires a robust cavity wall construction or additional acoustic insulation in the shared structure — a cost that is easy to underestimate at design stage.
Fire safety
Sleeping accommodation requires smoke and heat detectors, adequate escape routes, and — where the annexe is connected to the main house — fire separation between them. Building control will require a fire safety strategy for attached annexes in particular.
Structural requirements
Foundations must be designed by a structural engineer. In London, clay subsoil requires careful assessment — minimum 1m depth is typical, with some sites requiring 1.5m or deeper depending on proximity to trees. A structural engineer's input is essential before any ground works begin.
Free AI Assistant
Ask anything about your granny annexe
CIL Exemption for Residential Annexes
The Community Infrastructure Levy (CIL) is a charge that many London boroughs apply to new residential development. A self-contained annexe creates new residential floorspace and, in boroughs with a CIL charging schedule, would ordinarily trigger CIL liability. The charge can be substantial — some London boroughs charge over £200/m² for new residential development.
There is a specific CIL exemption for residential annexes built by owner-occupiers. The key conditions, verified from GOV.UK CIL guidance, are:
CIL residential annexe exemption: conditions
You must have a material interest in (own) the main dwelling at which the annexe is being built.
The main dwelling must be your sole or main residence. Buy-to-let landlords building annexes cannot claim this exemption.
The exemption must be applied for and obtained before commencement of development. If you start building without claiming the exemption, you lose it — there is no retrospective application route.
If you let the annexe or sell it separately from the main dwelling within three years of completion, the exemption is withdrawn and the full CIL charge becomes payable. The three-year period runs from the date of the building regulations compliance certificate for the annexe.
Practical step: The CIL exemption form (CIL Form 8) must be submitted to the council before any work starts — not at planning application stage. Ask your architect or planning consultant to manage this alongside the planning application so you do not miss the window.
Not all London boroughs have a CIL charging schedule — check whether your borough charges CIL before assuming it applies. Even in boroughs without CIL, the Section 106 regime may apply to annexes in some cases, though this is less common for standard residential annexes.
Council Tax on Annexes
A self-contained annexe is a separate dwelling for council tax purposes and is given its own council tax band. This means you pay council tax on both the main house and the annexe. The Valuation Office Agency determines the annexe's band based on its size, location, and facilities.
The 50% discount for relatives
Under The Council Tax (Reductions for Annexes) (England) Regulations 2013 (in force from 1 April 2014), a 50% reduction in council tax is available on an annexe if it is occupied by a relative of the person liable to pay council tax at the main dwelling.
The definition of relative is broad and includes: spouse, parent, child, stepchild, grandparent, grandchild, brother, sister, uncle, aunt, nephew, niece, and equivalent extended family relationships.
Council tax discount: the two rates
Where any relative (as defined above) occupies the annexe as their sole or main residence, the council tax on the annexe is reduced by 50%.
Where a dependent relative occupies the annexe, the council tax is fully exempt (100% discount). A dependent relative is defined as: a person aged 65 or over, a person who is severely mentally impaired, or a person who is substantially and permanently disabled. This is the classic "granny annexe" scenario where the regulation is most beneficial.
Apply for the discount directly with your London borough council tax department. You will need to demonstrate the relationship and that the annexe is the relative's sole or main residence.
Mortgage and Insurance Implications
Your mortgage
Informing your mortgage lender of the annexe is a condition of most mortgage agreements. Lenders vary in how they respond: some simply note the annexe on the file, others require a revised valuation. A few lenders have restrictions on properties with self-contained annexes, particularly if the annexe could be let separately — which could technically constitute mixed residential/rental use.
If you are building an annexe to help fund the property by letting it, be clear with your lender at the outset. Many residential mortgages do not permit letting any part of the property without consent, and adding a lettable annexe may trigger a requirement to move to a different mortgage product.
Buildings insurance
Your existing buildings insurance policy must be updated to include the annexe. Most insurers will cover an annexe used by family members under the existing policy with a premium uplift. If the annexe is let — even occasionally — you may need a specialist policy covering both residential and rental use.
Contents insurance for the annexe occupant is typically the occupant's own responsibility, not covered under the main property policy. Clarify this with your insurer before the annexe is occupied.
Frequently Asked Questions
Do I need planning permission for a granny annexe in London?
If the annexe will be self-contained — with its own kitchen, bathroom, bedroom, and external entrance — planning permission is always required. There are no exceptions based on family occupancy or the size of the annexe. An incidental outbuilding (home office, gym, occasional guest space without self-contained facilities) may qualify as permitted development under Class E, but the moment it provides all the facilities for independent living, planning permission is needed.
Can I build a granny annexe under permitted development?
You can build a non-self-contained outbuilding under Class E permitted development if it meets the size, height, and position conditions and remains incidental to the main house. The GOV.UK technical guidance makes clear that Class E does not cover primary living accommodation such as a bedroom, bathroom, or kitchen. A self-contained annexe — regardless of who will live in it — needs planning permission.
How much does a granny annexe cost in London in 2026?
All-in costs inc VAT: garage conversion £25k–£50k; side or rear extension annexe (20–30m²) £60k–£120k; detached outbuilding annexe (20–35m²) £50k–£120k. A detached outbuilding adds £4,000–£12,000 for running services (electricity, water, drainage) across the garden. Add £3,500–£8,000 for architect fees and £528 for the planning application if needed.
Is there a council tax discount for a granny annexe?
Yes. Under The Council Tax (Reductions for Annexes) (England) Regulations 2013, a 50% council tax reduction applies where a relative of the main dwelling council tax payer occupies the annexe. Where the occupant is a dependent relative (aged 65+, severely mentally impaired, or substantially and permanently disabled), a 100% exemption (no council tax at all) applies. Apply to your London borough council tax department with evidence of the relationship and residency.
What is the CIL exemption for a granny annexe?
Owner-occupiers who build a residential annexe within their own home's curtilage can apply for a CIL exemption. You must own and live in the main dwelling as your principal residence. The exemption must be applied for before any work starts (CIL Form 8). If you let or sell the annexe separately within three years of completion, the exemption is withdrawn and the full CIL charge becomes payable.
What conditions do councils attach to annexe planning consents?
Most London councils attach occupancy conditions preventing the annexe from being sold or let separately from the main dwelling. A common condition restricts use to 'ancillary residential accommodation for members of the household of the main dwelling only.' These conditions run with the land and bind all future owners permanently.
Does a granny annexe need building regulations approval?
Yes — all habitable annexes need building regulations approval, regardless of planning status. Habitable space standards apply (minimum ceiling heights, ventilation, natural light). The 2026 insulation standards, fire safety requirements, and Part E sound separation (for attached annexes sharing a wall with the main house) all apply. Building regulations approval is needed even if the annexe qualifies as permitted development.
Will a granny annexe add value to my home?
A well-designed self-contained annexe typically adds value, particularly in London where multi-generational living demand is high. The value uplift depends on the quality of the design, size, and planning status — an annexe with a planning consent and no restrictive occupancy condition is worth more than one with a condition preventing separate letting. Some lenders value properties with annexes cautiously, so check mortgage implications before building.
Common Mistakes
Building self-contained accommodation assuming it is permitted development
The single most common and costly mistake. Many homeowners build a garden studio with a kitchenette and shower room, believing it is covered as an outbuilding under Class E because it is in the rear garden and under the height limit. It is not. Once it contains self-contained accommodation, planning permission was required. The consequence is an enforcement notice, potential demolition, and the difficulty of securing retrospective consent with an occupancy condition attached.
Missing the CIL exemption window
CIL Form 8 must be submitted and approved before work starts. Homeowners who start on site before claiming the exemption — even by a day — lose the right to claim it. In a borough with a high CIL rate, this can mean an unexpected bill of £10,000–£50,000 or more depending on the size of the annexe. Your architect should flag this at planning application stage, but confirm it yourself.
Underestimating the cost of running services to a detached annexe
A detached outbuilding needs mains electricity, usually a water supply, foul drainage to the existing system, and some form of heating. Running these services across a typical London garden — possibly under hard landscaping, raised beds, or structures — costs £4,000–£12,000 and is rarely budgeted for accurately in early estimates. Get a detailed utility survey done before committing to a detached structure.
Not notifying your mortgage lender or insurer
Building a self-contained annexe and failing to inform your mortgage lender is likely a breach of your mortgage conditions. Similarly, your buildings insurance policy will not cover the annexe automatically. These are low-effort, important steps that are frequently overlooked in the focus on planning and construction.
Planning for letting within the CIL three-year clawback window
Some homeowners build an annexe for a parent, claim the CIL exemption, and then — when the parent moves to care — decide to let it within the three-year period. This triggers the full CIL liability retrospectively. If there is any possibility you might let the annexe within three years of completion, take specialist advice before claiming the exemption.
Summary
A granny annexe is a worthwhile project for many London homeowners, but the planning rules demand careful navigation. The self-contained versus incidental distinction is the starting point: get this wrong and the project may need to be demolished or retrospective planning sought.
If the annexe will be self-contained, budget for a planning application and occupancy conditions. If you own and live in the property, claim the CIL exemption before you start on site. If a dependent relative aged 65 or over will occupy the annexe, apply for the 100% council tax exemption.
Costs in London 2026 range from £25k–£50k for a garage conversion to £60k–£120k for an extension or detached outbuilding. The biggest cost drivers are specification, the need to run services to a detached structure, and the kitchen and bathroom fit-out.
Related Articles
Ready to start?
Get your annexe planning check and cost estimate
Our AI checks your property address, planning constraints, and conservation area status, then gives you a realistic cost estimate for your annexe type.
Or email hello@mayfairstudio.co.uk · 07405 920944