Summary

Garden rooms have shifted in the last few years from glorified sheds to genuine working spaces — home offices, therapy rooms, gyms, music studios — occupied for hours at a time, year-round, in British weather. That change matters electrically. A shed that occasionally runs a strip light and a radio has a trivial load. A garden office that a client sits in for eight hours a day, heated to a comfortable working temperature through a UK winter, with a laptop, two monitors, a space heater or a wall-mounted AC/heat-pump unit running near-continuously, has a load profile closer to a small habitable room than to an outbuilding.

The common mistake is sizing the supply on socket count rather than on what will actually be drawing current at the same time. A garden office with "just a couple of sockets and a light" can still pull 15–20A continuously through a winter afternoon once the heating load is counted properly, because unlike a workshop's intermittent power-tool loads, office heating runs almost continuously for the whole occupied period. Undersizing the cable on the assumption that a home office draws "next to nothing" is one of the more common under-specification errors in outbuilding electrics.

This article focuses specifically on the demand calculation and cable sizing decision for garden room/office-type loads. For the general outbuilding supply design process — submain cable types, sub-consumer unit configuration, and the PME/TT earthing decision in full — see workshop supply and outdoor electrical, both of which this article assumes as background. What follows applies that design process specifically to the office/annexe load profile rather than the workshop/tool load profile.

Key Facts

Quick Reference Table

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Garden room use Typical diversified demand Submain (up to ~25m) Submain (25–40m) Sub-CU ways
Garden office — laptop, lighting, 1.5kW heater 10–13A 2.5mm² SWA 4mm² SWA 3-way
Garden office — desktop + monitors, 2kW heater 14–18A 4mm² SWA 6mm² SWA 4-way
Garden office with AC/heat pump (2.5–3.5kW) 16–22A 4mm² SWA 6mm² SWA 4–5-way
Music/therapy studio — lighting, sockets, heating, no AC 12–16A 4mm² SWA 4–6mm² SWA 4-way
Garden annexe/bar — heating + kettle/microwave/fridge 20–26A 6mm² SWA 6–10mm² SWA 5–6-way
Garden gym — lighting, sockets, dehumidifier, no heavy plant 12–16A 4mm² SWA 4–6mm² SWA 4-way

Detailed Guidance

Step 1: Build the load schedule properly

Start by listing every load that could realistically be on at the same time, not every socket that exists. The distinction matters because a garden office has two very different categories of load with different diversity treatment.

Continuous/near-continuous loads (apply little or no diversity):

Intermittent loads (diversity can reasonably be applied):

The IET On-Site Guide diversity tables (Table 1A) are written primarily around domestic ring-final and lighting circuits inside a house; they don't have a specific "garden office heater" line. The safer approach for a single-room occupied building is to size the heating and IT load at close to 100% of rated current and apply diversity only to genuinely intermittent items — a conservative approach costs one cable size, an under-sized submain costs a callback and a burnt connection.

Step 2: Worked example — garden office with AC and desktop workstation

Scenario: A 3m × 4m insulated garden office, 32m cable run from the house consumer unit, used as a full-time home office. Loads: 2.5kW wall-mounted heat pump/AC unit (heating and cooling), desktop PC + 2 monitors (~350W), LED lighting (~60W), general sockets for phone charger/printer (~100W intermittent).

Load Rated current (230V) Diversity applied Diversified current
AC/heat pump unit (2.5kW) 10.9A 100% (near-continuous) 10.9A
Desktop + 2 monitors (350W) 1.5A 100% (continuous use) 1.5A
LED lighting (60W) 0.26A 90% 0.23A
General sockets (printer, charger etc.) up to 13A available 25% (intermittent) 3.25A
Total diversified demand ~15.9A

Round up for headroom: specify a 20A submain protective device and select cable on that basis, keeping a spare way in the sub-CU for future load growth (a second AC unit, a kiln, an espresso machine — garden room uses tend to grow over time). This 2.5kW-unit scenario sits at the low end of the Quick Reference Table's "AC/heat pump (2.5–3.5kW)" row (16–22A) — a larger 3.5kW unit with the same other loads pushes diversified demand toward the upper end of that range.

Cable check (4mm² 3-core SWA, 32m run):

Result: 4mm² 3-core SWA on a 20A submain protective device comfortably serves this load with margin for the AC unit's startup current and future expansion. Where the office also gets a kitchenette or a second heavy load later, step up to 6mm² at design stage rather than retrofitting.

Step 3: When to step up to 6mm² or beyond

Three situations push the design past a straightforward 4mm² submain:

Step 4: The earthing decision applies exactly as for a workshop

The load calculation is specific to garden room/office use, but the earthing decision is not — it follows the same BS 7671:2018+A4:2026 Regulation 411.4.5 logic covered in full in workshop supply. In summary for a garden office: if the building has any external metalwork a person could touch while standing on the ground outside (a metal door frame, external light fitting on a metal bracket, metal cladding, a rainwater downpipe bonded to structure), exporting the PME earth risks a touch-voltage hazard if the supply neutral fails, and a local TT earth with a whole-supply 30mA RCD is the correct, more conservative specification. Timber-clad garden offices with no exposed extraneous-conductive parts are the case most likely to be judged as PME-exportable, but many installers specify TT as standard practice for detached buildings regardless, because the consequence of getting the judgement wrong is severe and the additional cost of a TT electrode is modest.

Step 5: Circuit design inside the garden room

A garden office deserves the same circuit separation as a small extension, not a single unprotected spur run from the house:

Frequently Asked Questions

Is a 16A submain enough for a garden office?

Only for the lightest loads — lighting, a laptop, and a small (under 1.5kW) heater with no AC. As soon as the office includes a wall-mounted AC/heat-pump unit, a desktop workstation used all day, or any kitchenette equipment, the diversified demand commonly reaches 15–20A, and specifying only 16A leaves no headroom for the unit's startup current or future load growth. Calculate the actual diversified demand rather than defaulting to 16A because it "sounds like plenty for a shed."

Does the electric heater in a garden office need its own circuit?

It's strongly recommended, even where not strictly mandatory. Sharing a heater with the socket circuit means a trip on either takes both out — awkward when the heater fails on a cold day and the whole room, including the computer equipment, goes dark. A dedicated circuit with its own RCBO isolates faults to the load that caused them.

Can I just run an extension lead to the garden office instead of a proper submain?

No. This is not compliant for a permanent structure and is a well-known fire and shock risk — extension leads run through doors, over damp ground, and are not rated for continuous outdoor exposure or the sustained current a garden office draws. A proper SWA submain to a dedicated circuit, installed and certified to BS 7671:2018+A4:2026, is the only acceptable approach for a permanent garden building with power.

Does a garden office need Part L thermal compliance, and does that affect the electrical design?

Thermal performance is governed separately by Building Regulations (Part L, where applicable to the specific project) and by the client's own insulation specification, but it directly affects the electrical load: a poorly insulated garden office needs a significantly larger heater to hold temperature, which increases the diversified demand and can push the submain up a cable size. Establish the building's insulation standard before finalising the heating load and the cable size — retrofitting a bigger heater later often means uprating the submain too.

Regulations & Standards