Summary
A granny annexe sits at the expensive end of garden buildings because, unlike a garden office or studio, it needs the full complement of habitable-space services: kitchen, bathroom, sleeping accommodation, heating, and (usually) independent access. That takes it out of the simpler "garden room" pricing bracket covered in garden room pricing guide and into something closer to a small self-contained house — with the Building Regulations, drainage, and utilities cost that implies.
The single biggest pricing variable is planning status, and it's worth spending time with the client on this before pricing anything. An annexe that is genuinely ancillary to the main house — no separate postal address, sharing utilities or clearly dependent on the main dwelling, occupied by a family member rather than let independently — can often proceed as permitted development if it meets the size/height/coverage conditions in outbuildings regs. An annexe intended to function as an independent dwelling (own utilities, could be let separately, own access) is legally a separate dwelling and needs full planning permission — a materially different, longer, costlier project. Price the wrong one and either the client gets an enforcement notice, or you've quoted a full-planning job at ancillary-annexe money and lost margin on every line.
The other major cost swing is foundation and construction method. A lightweight timber-frame annexe on a raft or screw-pile foundation is faster and cheaper than a masonry cavity-wall build on strip foundations, but the masonry route is often preferred where the annexe needs to visually match the main house or where longevity/resale value matters more than speed.
Key Facts
- Planning route determines scope, not just fee — PD-compliant ancillary annexe: no planning application, but still subject to Building Regs if used for sleeping (Building Regs always apply to sleeping accommodation regardless of size, per outbuildings regs); independent-dwelling annexe: full planning application (8–10 week decision) plus full Building Regs compliance.
- Size threshold — annexes under 15m² are otherwise exempt from Building Regs, but the moment sleeping accommodation is included, Building Regs apply regardless of floor area — this catches out clients who've read the general "15m² exempt" outbuilding rule without realising the sleeping-accommodation override.
- Foundation options — concrete raft (fast, good for level sites, £90–£150/m²), strip/trench-fill footings (for masonry, £110–£180/m² of footprint), screw piles (fastest, best for uneven ground or minimal disruption, £70–£120/m²).
- Construction method cost band — timber-frame (SIPs or open-panel) £1,800–£2,600/m² turnkey; masonry cavity wall £2,200–£3,200/m² turnkey; the masonry premium buys longevity, sound insulation, and closer material match to the main house.
- Utilities — a genuinely self-contained annexe needs its own water supply branch (with stopcock), foul drainage connection (gravity where possible, macerator/pump where not — see below), electrical supply (dedicated consumer unit, SWA armoured cable run, Part P notifiable), and usually its own heating source (electric, or extended from the main house's system).
- Foul drainage — where gravity connection to the main soil stack or a manhole isn't achievable, a macerator system (Saniflo-type, to BS EN 12050-1) adds £600–£1,800 but removes the need for expensive deep excavation to achieve fall.
- Bathroom fit-out — budget £3,500–£7,000 for a compact wet room or shower room within the annexe footprint (see wet room installation guide for tanking/fall detail if going the wet-room route).
- Kitchen fit-out — a compact galley kitchen (annexe-scale) runs £2,500–£6,000 depending on units, worktop, and appliance specification.
- Insulation/Part L — annexes must meet current Part L standards as a new build (or new-build-equivalent for a PD ancillary structure used habitably) — typically 0.18 W/m²K walls, 0.13–0.16 W/m²K roof — driving a higher-spec insulation package than a simple garden office.
- Accessibility — many granny annexes are commissioned specifically for an elderly or mobility-limited relative; Approved Document M and level-access/wet-room detailing (see accessible bathrooms) should be priced in from the outset, not retrofitted.
- Council tax — a fully self-contained annexe with its own amenities can attract a separate council tax band, though a 50% discount applies in England where it's occupied by a family member and used in connection with the main house (Annexe Discount) — flag this to the client as a running cost, not a build cost, but it affects the business case for the project.
- Programme — PD ancillary annexe: 10–16 weeks on site after a straightforward Building Control process; full-planning independent-dwelling annexe: 6–9 months total including the planning decision period.
Quick Reference Table
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Try squote free →| Annexe Type | Size | Construction | Typical Turnkey Cost |
|---|---|---|---|
| Basic ancillary annexe (bedroom + shower room, no kitchen) | 15–20m² | Timber-frame, raft foundation | £28,000–£45,000 |
| Standard granny annexe (bed, bath, kitchenette) | 25–30m² | Timber-frame, raft/screw-pile | £48,000–£75,000 |
| Standard granny annexe (bed, bath, kitchenette) | 25–30m² | Masonry cavity wall, strip footings | £58,000–£90,000 |
| Larger annexe (bed, bath, kitchen, living space) | 35–45m² | Timber-frame | £75,000–£115,000 |
| Larger annexe (bed, bath, kitchen, living space) | 35–45m² | Masonry, matched to main house | £90,000–£140,000 |
| Independent-dwelling annexe (full planning) | 30–40m² | Masonry, full Building Regs | £100,000–£160,000+ |
Detailed Guidance
Establishing planning status before pricing anything
Ask directly: who will live there, is it a family member dependent on the main household, will it have its own postal address, could it be let separately or sold separately from the main house? The answers determine which of two very different projects you're pricing:
- Ancillary annexe (PD route possible) — occupant is dependent on the main house (meals, care, or general household connection), not let separately, ideally sharing or connected to main-house utilities where practical. Check against the full PD conditions in outbuildings regs (size, height, boundary distance, coverage).
- Independent dwelling (full planning required) — self-sufficient unit that could function as a separate letting, own utilities, own access not through the main house. This is legally a new dwelling and needs full planning permission — budget an extra £3,000–£6,000 in professional fees (architect, planning consultant, structural engineer) on top of the build cost, and 8–10 weeks minimum for the planning decision before a spade goes in the ground.
Get this classification agreed with the client's local planning authority (or a planning consultant) in writing before pricing the job on the assumption of PD — an enforcement notice after completion is a cost nobody in the chain wants to absorb.
Pricing the foundation and structural shell
| Element | Timber-Frame | Masonry |
|---|---|---|
| Foundation | Raft (£90–£150/m²) or screw pile (£70–£120/m²) | Strip/trench-fill footings (£110–£180/m² footprint) |
| Shell erection time | 1–2 weeks (panelised) to 3–4 weeks (site-built) | 4–6 weeks |
| Weathertight milestone | Faster — panels arrive largely pre-insulated | Slower — brick/block, cavity, roof all sequential |
| Match to main house | Requires cladding/render finish to match | Natural match if using same brick/block |
| Typical shell cost (structure + roof, ex fit-out) | £900–£1,300/m² | £1,100–£1,600/m² |
Pricing utilities — the cost block clients underestimate
Utilities are where inexperienced quotes fall apart, because clients think of an annexe as "a small extension" and forget it needs the same services as a full house in miniature:
- Water — new branch off the main supply or a separate MDPE run from the boundary; £600–£1,800 depending on run length and whether trenching crosses paving/driveway.
- Foul drainage — gravity connection where fall allows: £800–£2,000; macerator/pump system where it doesn't: add £600–£1,800 to that.
- Electrical — dedicated consumer unit and SWA cable run from the main house distribution board (or a new supply from the DNO for larger annexes): £1,500–£3,500.
- Heating — electric panel heaters/infrared (cheapest to install, higher running cost): £600–£1,500; extending wet central heating from the main house (better running cost, needs pipe run and possibly boiler capacity check): £2,000–£4,500; standalone system (small combi or heat pump for the annexe): £3,500–£7,500.
Worked example — 28m² ancillary annexe, timber-frame, PD route
Detached bungalow rear garden, occupant is client's elderly parent, annexe connects to main house's water/drainage where the run is short, dedicated electrical supply.
- Foundation (concrete raft): £3,200
- Timber-frame shell (SIPs panel system, roof, windows/doors): £29,500
- Insulation and internal lining to Part L: included in shell figure
- Water connection (short run to main supply): £950
- Foul drainage (gravity, short run to existing manhole): £1,600
- Electrical (dedicated consumer unit, SWA run, first + second fix): £2,800
- Heating (electric panel heaters, 4 rooms): £1,100
- Bathroom fit-out (compact wet room): £4,800
- Kitchenette fit-out (galley, 6 units + appliances): £3,600
- Flooring, decoration, second fix joinery: £3,200
- Sub-total cost: £50,750
- 18% margin/overheads: £9,135
- Net price: £59,885
- VAT (20% — standard-rated; this is an ancillary annexe sharing the main house's utilities and access, so it does not qualify for the new-dwelling zero-rating under VAT Notice 708): £11,977
- Quoted price: £71,862 inc. VAT
Worked example — 32m² independent-dwelling annexe, masonry, full planning
Detached house, adult child's family, annexe designed with own access gate, own postal-capable address, full self-sufficiency (client's stated intent to potentially let it in future).
- Planning application + architect drawings + planning consultant: £5,200
- Structural engineer calcs (masonry, roof structure): £1,400
- Foundations (strip footings, masonry-standard): £5,800
- Masonry shell (cavity wall matched brick, roof): £48,000
- Water, foul drainage (independent connection with own inspection chamber), electrical (new DNO supply assessed): £9,500
- Heating (standalone combi boiler system): £4,200
- Bathroom + kitchen fit-out (full spec): £13,500
- Flooring, decoration, second fix: £6,800
- Sub-total cost: £94,400
- 20% margin/overheads: £18,880
- Net price: £113,280
- VAT — two possible outcomes, and getting this wrong is a five-figure pricing error either way:
- Standard-rated (20%): £135,936 inc. VAT — applies if the planning permission carries a condition preventing separate sale or letting from the main house, which disqualifies zero-rating regardless of how self-contained the annexe is built
- Zero-rated: £113,280 (no VAT charged) — only applies if the annexe qualifies as a genuinely new, self-contained dwelling under VAT Notice 708 §3 (own entrance, no internal access to the main house, no planning condition restricting separate use or disposal) — confirm eligibility with an accountant or HMRC before quoting a zero-rated price, since VAT under-charged in error is the contractor's liability to HMRC, not the client's
Frequently Asked Questions
Can I quote a granny annexe as permitted development to save the client money, even if it's really an independent dwelling?
No — and doing so exposes both you and the client to enforcement risk. If the intended use is genuinely independent (own access, own utilities, potential to let separately), it's a separate dwelling in planning terms regardless of size, and requires full planning permission. Quote it correctly; a cheaper-looking price built on the wrong planning assumption isn't a favour to the client.
Does a granny annexe need its own EPC?
If it's assessed as a separate dwelling (independent-dwelling route), yes — it needs its own Energy Performance Certificate on completion, same as any new dwelling. An ancillary annexe that remains legally part of the main house typically doesn't need a separate EPC, but check current guidance with Building Control at the point of completion sign-off.
How much does the Annexe Discount on council tax affect the pricing conversation?
It's a running-cost consideration, not a build-cost one, but worth raising with the client early — in England, a 50% council tax discount applies to an annexe occupied by a family member and used in connection with the main house. This can be a meaningful factor in whether the client proceeds with the ancillary route versus a fully independent unit, so it belongs in the initial scoping conversation even though it doesn't change your quote.
Is a modular/prefab annexe cheaper to quote than a site-built one?
Often faster, not necessarily cheaper once foundations, utilities, and finishing are included — modular suppliers frequently quote the unit cost only, excluding groundworks and services connection, which can add 25–40% to the advertised "from" price. If subcontracting a modular supplier, make sure your quote to the client includes the full turnkey scope, not just the unit delivery.
Regulations & Standards
GPDO 2015 Schedule 2, Part 1, Class E — permitted development for outbuildings; see outbuildings regs for the sleeping-accommodation override and coverage limits.
Building Regulations Parts A, B, C, F, H, L, M, P — full compliance required for any habitable annexe regardless of PD/planning route once sleeping accommodation is present.
BS EN 12050-1 — macerator/lifting pump systems for foul drainage where gravity fall isn't achievable.
BS 7671:2018+A4:2026 — electrical installation standard for the dedicated supply.
The Council Tax (Prescribed Classes of Dwellings) (England) (Amendment) Regulations 2012 — basis for the 50% Annexe Discount, in force from 1 April 2014.
NHBC — Guidance on annexe and outbuilding construction standards
outbuildings regs — permitted development conditions, sleeping-accommodation Building Regs override, planning tests for ancillary vs independent use
garden room pricing guide — pricing model for non-habitable garden buildings, useful comparison baseline
accessible bathrooms — Approved Document M detailing often specified for annexes built for elderly relatives
macerator systems — macerator/pump drainage detail referenced in the utilities section above