The triangular void beneath a staircase is one of the most under-used spaces in a UK home. Whether you have a simple open triangle or a boxed-in cupboard, that space can become a fitted storage unit, a working cloakroom toilet, a home-office nook, or a utility area — often without planning permission and with a relatively modest budget.
This guide covers the most popular conversions, what they cost, and the rules that apply.
What Can You Do With Understairs Space?
The geometry of your staircase dictates the options. A straight staircase in a narrow hallway typically offers 1.5–2.5 m³ of usable volume. Dog-leg or quarter-turn stairs may give more. The key constraint for a toilet is headroom — you need at least 1.9 m at the standing point (ideally 2.0 m) to satisfy Building Regulations Part K (protection from falling), and enough depth for the WC and cistern (typically 750–800 mm from wall to front of pan).
Costs at a Glance
| Conversion Type | Typical Cost (Supply & Fit) |
|---|---|
| Basic fitted shelving / pull-out drawers | £800–£1,800 |
| Bespoke joinery fitted storage (painted MDF or solid wood) | £1,800–£3,500 |
| Cloakroom WC — basic (close-coupled WC, basin, boxing) | £2,500–£3,500 |
| Cloakroom WC — mid-range (wall-hung WC, concealed cistern, tiled) | £3,500–£5,500 |
| Cloakroom WC — high-spec (bespoke joinery, feature tiling, designer sanitaryware) | £5,500–£9,000+ |
| Home-office nook (desk, shelving, power, lighting) | £1,200–£2,800 |
Costs above are London and South East rates; expect 10–20% less elsewhere in England.
Fitted Storage Under Stairs
The simplest conversion. A carpenter or joiner frames up an enclosure if one doesn’t already exist, adds a door, and fits internal shelving, pull-out drawers, or a combination of both.
Pull-out drawer systems are the most practical for deep triangular spaces — drawers mounted on full-extension runners allow access to the full depth without reaching in. Budget £1,500–£3,000 for a quality fitted system with soft-close drawer boxes.
Open shelving works well in hallways where the style is more casual or where the homeowner wants easy access. Wine storage, shoe racks, and sports equipment holders are popular configurations.
If you’re adding lighting, a single LED strip or downlight on a PIR sensor makes the space much more functional. This is a minor electrical addition and is unlikely to require notification under Part P if connected to an existing circuit by a competent person — but a registered electrician should confirm.
Understairs Cloakroom (WC)
Adding a downstairs toilet under the stairs is one of the most-valued home improvements in the UK, adding convenience and, in most estate agents’ views, measurable resale value. The key constraints are space and drainage.
Minimum Space Required
The British Standard BS 8300 and Building Regulations Part M (access) set minimum dimensions for a WC compartment:
- Minimum internal floor area: approximately 900 mm × 1,500 mm for a conventional close-coupled WC and small basin.
- Preferred minimum: 1,000 mm × 1,700 mm to allow easier use and comply with Part M accessibility guidance.
- Wall-hung WC with concealed cistern: saves 150–200 mm in depth, useful in tight spaces.
If headroom under the stairs is limited, a slimline wall-hung WC and a countertop basin on a floating vanity unit are the most space-efficient combination.
Drainage — The Critical Factor
Getting the toilet waste to connect to the soil stack is the most technically demanding part of the project. Options include:
Gravity drainage — if the staircase is adjacent to or close to an external wall with an existing soil stack, a 110 mm soil pipe can usually be run at sufficient fall (minimum 1:40) to connect. This is the preferred and most reliable method.
Macerator (Saniflo-type) pump — where gravity drainage isn’t feasible (e.g., the stairs are at the centre of the house), a macerator pump macerates waste and pumps it to the stack through a 22–32 mm pipe, which can run horizontally for up to 12 m (check manufacturer’s specs). Macerators cost £400–£900 for the unit; installation adds £300–£600. They require an electrical connection and have ongoing maintenance requirements (blade replacement every 5–10 years or so).
Ventilation
Building Regulations Part F requires mechanical ventilation in internal WCs — typically a 100 mm extractor fan ducted to an external wall or roof void. The duct run must not exceed 6 m equivalent length.
Do You Need Building Regulations Approval?
Yes, in nearly all cases. Adding a WC involves:
- Drainage connections (Part H)
- Ventilation (Part F)
- Electrical installation (Part P) for extractor and lighting
- Structural considerations if any stair trimmer joists are disturbed
Your plumber or builder should arrange a Building Regulations application (full plans or building notice) with your local authority. The inspector will visit to check drainage falls and ventilation before the works are closed in.
Planning Permission
Not normally required for an internal WC conversion. The only exception would be a listed building, where listed building consent may be needed even for internal alterations.
Home-Office Nook
For those who work from home part-time, the understairs void can fit a compact desk (typically 600–800 mm depth), overhead shelving, and a power outlet. This works best where the opening is at least 900 mm wide and headroom reaches 1.9 m at the seat position.
Add a sliding or barn door if you want to close it off. Good task lighting (recessed downlight at the front of the cupboard aperture) makes the space usable. USB-C power sockets flush-mounted to the desk surface are a practical detail.
Home-Office vs. Cloakroom: Which Adds More Value?
In a 3-bed house without a downstairs WC, adding a cloakroom is likely the higher-value investment — a ground-floor toilet is consistently cited by estate agents as increasing resale appeal. In a home that already has a cloakroom, a well-fitted study nook is a more practical upgrade.
Finding a Tradesperson
A storage conversion is a carpenter’s job. A cloakroom conversion requires a plumber (ideally with Gas Safe registration if any heating is nearby, though not mandatory for WC-only work) and a Part P-registered electrician for extractor and lighting. Many bathroom companies offer a design-and-fit service that covers all trades.
Get three quotes and ask specifically whether Building Regulations applications are included in the price — some contractors leave this to the homeowner.
