How Do You Install an Outside Tap? Water Regulations, Backflow Prevention, and Pipe Sizing

Summary

An outside tap is one of the most common plumbing additions to UK homes, yet it is one of the most frequently installed incorrectly. The Water Supply (Water Fittings) Regulations 1999 require notification for outside taps because a hose connected to the tap creates a potential pathway for contamination to enter the mains water supply through backflow — the reverse flow of water when mains pressure drops.

The regulation requires a double check valve (DCV) to be installed as close as possible to the point where the supply tee is taken from the cold water main. This provides two independent non-return check valve seats, providing protection against Category 3 fluid risk (garden hose water, chemicals, fertilisers). A single check valve does not meet the requirement for an outside tap application.

This article covers the installation requirements for a straightforward domestic outside tap supply. Commercial installations, irrigation systems connected to chemical dispensers, and swimming pool fill connections require higher levels of backflow protection and should be assessed separately.

Key Facts

• Notification required — outside taps are listed in Schedule 2 of the Water Fittings Regulations 1999; you must notify the water supplier before installation (or use a WaterSafe-approved contractor who self-certifies)
• Double check valve — Category 3 fluid risk; single check valve is not sufficient; DCV must be WRAS-approved
• DCV position — install the DCV as close as possible to the supply tee, ideally inside the property before the wall penetration
• Hose union bib tap — the correct fitting for an outside tap; has a 3/4" BSP male thread and an integral hose union
• 15mm supply — standard supply pipe size for a single outside tap; 22mm if serving a long run (>15m) or multiple outlets
• Drainable isolating valve — fit inside the property; allows the supply to be isolated and the external pipework drained for frost protection; look for a valve with a drain screw
• Frost protection — pipe exposed on external wall should be insulated with lagging; a drainable valve inside is the primary protection mechanism
• Wall entry — pipe passes through the wall in a sleeve (typically 28mm plastic conduit or rubber grommet); seal around the sleeve externally with silicone
• Outside tap position — typically mounted 300–500mm above ground level; surface-mounted back plate or wall box available
• WRAS-approved fittings — all fittings in contact with potable water must be WRAS-approved; check the WRAS product database
• Separate supply — tee off the rising main or cold distribution pipe inside; do not tee off the hot water system
• Combi boiler cold supply — the cold feed to the kitchen tap (direct mains) is the appropriate tee-off point; do not tee from indirect cold feeds

Quick Reference Table

Detailed Guidance

Notification and WaterSafe

Before installing an outside tap, you must either notify your water supplier (most have an online notification form) or use a WaterSafe-approved plumber who is authorised to self-certify the work. Notification is a legal requirement under Schedule 2 of the Water Supply (Water Fittings) Regulations 1999, which lists "a tap to which a hose pipe or similar fitting can be attached" as notifiable work.

In practice, many homeowners install outside taps without notifying their water supplier, and enforcement is rare. However, the requirement exists to protect public health — backflow from hoses has caused real contamination events — and a plumber installing without notification is working outside the regulations.

WaterSafe is the UK's register of approved plumbers for water fitting work. A WaterSafe member can self-certify the work, providing the homeowner with a certificate of compliance equivalent to a Building Regulations completion certificate.

Supply Tee and Route

Take the supply from the cold water main (rising main) or the mains-pressure cold supply to the kitchen — not from any storage-fed indirect circuit. On combi boiler systems, all cold water is mains-fed, so tee off any cold supply in the property.

Route the supply pipe to the external wall in 15mm copper or plastic. WRAS-approved plastic systems (such as JG Speedfit or Hep2O) are perfectly suitable and are easier to route through walls. Avoid excessive bends and keep the run as short as possible to minimise pressure drop.

On the internal side of the external wall, install the components in order from the supply:

1. Isolating/service valve (with drain screw)
2. Double check valve
3. Wall penetration sleeve

Do not fit these components on the external face of the wall — frost can prevent them from operating correctly, and the DCV particularly must be accessible for maintenance.

Wall Penetration

Drill through the external wall at the intended tap position using a core drill or long SDS bit. A 28mm hole is typically sufficient for 15mm pipework in a sleeve. Push a length of plastic conduit or heavy-duty rubber grommet through the hole. This protects the pipe from abrasion by the masonry and allows it to be withdrawn for maintenance.

The hole must slope slightly downwards from inside to outside (approximately 5mm fall) so any condensation or water ingress drains outwards, not into the wall cavity. In a cavity wall, ensure the pipe passes through the cavity without touching the insulation — bridge across a short length of pipe rather than leaving it in contact with cavity insulation.

Seal around the conduit on the external face with UV-stable silicone sealant. Do not seal on the internal face — this should be left accessible. Fit an escutcheon plate on the internal face to neaten the penetration.

Fitting the Outside Tap

The hose union bib tap mounts directly to a back plate screwed to the wall, or inside a plastic wall box with a removable lid. The back plate or wall box protects the fitting from mechanical damage and provides a neater appearance.

The bib tap has a 3/4" BSP male thread. Connect to the 15mm supply using a reducing compression elbow or by directly threading onto the bib tap body — some back plates include a fitted 15mm compression inlet.

Position the tap 300–500mm above ground level — low enough to be accessible, high enough to clear flower borders and not be damaged by garden machinery. If fitting adjacent to a water meter or stopcock box, ensure adequate clearance for the meter reader.

Frost Protection

A drainable isolating valve fitted internally is the primary frost protection. In winter, turn off the isolating valve and use the drain screw to drain down the external pipework. The drain screw opens a small port that vents the pipe to atmosphere; with the supply isolated, any water in the external pipework will drain out of the tap.

Additionally, insulate any pipework on the external face of the wall — even a short section of exposed copper between the wall penetration and the tap body can freeze and split the pipe. Pre-slit foam pipe lagging secured with UV-stable tape provides reasonable protection.

Self-draining (frost-protected) outside tap fittings are available. These automatically drain when the tap is turned off, eliminating the need to manually isolate and drain. They are more expensive but virtually eliminate the risk of frost damage.

Testing the Installation

On completion, turn on the supply and check all joints for leaks. Test the double check valve by closing the tap and checking for backflow through the DCV — use a backflow check kit or test procedure from the DCV manufacturer's data sheet. Confirm that the isolating valve seals correctly and that the drain screw allows the external pipework to drain fully.

Frequently Asked Questions

Do I need planning permission for an outside tap?

No. Outside taps are permitted development and do not require planning permission. However, notification to the water supplier is required under the Water Fittings Regulations — this is separate from planning.

Can I just fit a single check valve instead of a double check valve?

No. The Water Fittings Regulations specify that a hose union tap constitutes a Category 3 fluid risk, and Category 3 requires a double check valve as a minimum. A single check valve only provides Category 2 protection. Using a single check valve makes the installation non-compliant.

Does it matter where the double check valve goes in the pipework?

Yes. The DCV should be positioned as close to the supply tee as practicable, before the wall penetration. Installing it on the outside of the building is not acceptable — it must be accessible, inside the property in a frost-free location. The principle is to protect the mains supply as early as possible.

Can I use plastic pipe for an outside tap installation?

Yes. WRAS-approved plastic push-fit systems (JG Speedfit, Hep2O, etc.) are fully acceptable for outside tap installations. Ensure any plastic pipe on the external wall is UV-resistant or protected from sunlight — standard polybutylene pipe degrades under prolonged UV exposure.

My outside tap has frozen and burst — what do I do?

Turn off the isolating valve immediately. Allow the burst section to thaw before attempting to cut out and replace. In future, always use the isolating valve and drain screw at the end of the season. If your current valve does not have a drain facility, replace it with one that does.

Regulations & Standards

• Water Supply (Water Fittings) Regulations 1999 (SI 1999/1148) — notifiable works Schedule 2; backflow prevention requirements

• BS EN 13959 — double check valve assemblies for backflow prevention

• WRAS Water Regulations Guide (WRAS, 2021 edition) — detailed guidance on compliance for installers

• Water Industry Act 1991 — underpins the Water Fittings Regulations; offences for non-compliant installations

• WRAS Water Regulations Guide — comprehensive guidance on Water Fittings Regulations compliance

• WaterSafe — Find an Approved Plumber — register of approved plumbers; homeowner notification information

• Plumbing Manufacturers International — Backflow Prevention — technical guidance on check valve selection

• Anglian Water / Thames Water — Notification forms — example water supplier notification procedures

• copper soldering — making permanent copper pipe joints for the supply

• water regulations — overview of Water Fittings Regulations for plumbers

• frost protection — protecting pipework from freezing in winter