Armoured cable — specification, route options and costs
The standard supply cable for a garden office is Steel Wire Armoured (SWA) cable, typically 6mm² twin-and-earth (two-core plus earth) for a sub-main supply up to 32A, or 10mm² for higher load applications (electric heating, air-source heat pump). SWA cable is specified because it is mechanically robust against accidental damage (spades, ground movement), weatherproof, and rated for direct burial. The cable runs from the house main consumer unit (or a dedicated outdoor supply fuse way) to the garden office consumer unit. Route options: underground burial (recommended) — trench excavated to 450mm depth minimum in un-trafficked garden areas (500mm minimum under hard landscaping), cable laid on a sand bed, marked with cable route tiles, backfilled. Overhead cable is permitted for spans under 3.5m without intermediate support, but is rarely used for garden offices due to aesthetic concerns and weather exposure — span masts can be used for longer overhead runs but add cost and planning considerations. Trench excavation through a typical London garden (15–25m cable run): £300–£600 for labour, £150–£300 for backfill sand and cover tiles. Cable material: 6mm² SWA at 15–25m costs £80–£150. Total cabled trench cost before electrical connections: £550–£900. Professional electrician for terminations, consumer unit installation and EIC certification: £900–£1,800. Combined: £1,500–£2,700 for a basic 20–25m run.
Consumer unit specification and circuit design for a garden office
The garden office should have its own dedicated consumer unit (CU) — not a spur off a domestic ring main. A dedicated CU provides: independent isolation (switch off the office supply without affecting the house), overcurrent protection for each circuit (RCBOs on modern compliant boards), and a clear circuit identification record. For a standard London garden office up to 25m², a 6-way CU (Hager VML206P or Wylex NHBM6M equivalent) with RCDs and MCBs provides adequate capacity. Recommended circuits: 32A ring main for double sockets (power, monitors, desk equipment); 6A lighting circuit (LED downlights or track, typically 200–400W total load); 16A radial for electric heating (1.5–2kW panel heater or 1.8kW heated floor); 10A radial for USB/data/A/V; optional 32A radial for air conditioning (if split HVAC unit installed). A 10-way CU (Hager VML210P) is recommended for offices with heat pump, air conditioning or EV charging. The CU in the garden office must be RCD-protected — typically via a 63A RCD in the house CU feeding the sub-main, providing fault protection for the entire garden office installation. All CU work and circuit installation must be Part P notified or self-certified by a registered competent person.
Data connection — CAT6 ethernet vs Wi-Fi for London home offices
A dedicated hardwired CAT6 ethernet connection is significantly more reliable than Wi-Fi extension for a garden office, particularly in London where dense urban Wi-Fi environments cause interference and speed degradation. CAT6 supports 1Gbps speeds at up to 100m, more than sufficient for any garden office application. The cable is best run alongside the SWA power cable in the same trench, in a separate 20mm conduit (data and power cables should not share the same conduit due to electromagnetic interference). A single CAT6 run (20–25m) costs £30–£80 in materials and adds £100–£200 in additional labour time — exceptional value for the bandwidth reliability improvement. At the office end, a small 5-port managed or unmanaged switch (TP-Link, Netgear) at £20–£60 provides multiple ethernet ports for desktop computers, printer, NAS and smart displays. Wi-Fi access points can then be added inside the office using the hardwired connection as backbone — far more reliable than extending the main house network via mesh or powerline adaptors. CAT6 vs fibre: for runs under 100m, CAT6 is indistinguishable from fibre in performance and costs a fraction of the price. Fibre is rarely justified for single-office applications.
Part P compliance, DNO requirements and cost summary
All electrical installation work in a garden office connected to a domestic supply must comply with Part P of the Building Regulations and BS 7671:2018 (18th Edition IET Wiring Regulations). The practical route is to use an NICEIC or NAPIT-registered electrician who self-certifies the work — they lodge a notification with the local authority building control and provide an Electrical Installation Certificate (EIC) on completion. The EIC is a valuable legal document; it confirms compliance, is required by mortgage lenders and conveyancing solicitors, and provides protection if any insurance claim arises from an electrical fault. DNO (Distribution Network Operator) notification: for domestic garden office connections under 100A (virtually all residential garden offices), no DNO notification or approval is required. The supply is simply a sub-main from the existing house connection. If the combined total house and garden office load would require a service upgrade (house currently on 60A supply, garden office adding significant load), a DNO application may be required — this is rare and usually avoided by specifying a consumer unit with adequate capacity. Cost summary (2025–2026 London rates): basic connection 20m trench, 6-way CU, 4 circuits, EIC: £1,500–£2,500. Full spec 25m trench, 10-way CU, 6 circuits, CAT6, external PIR light, EIC: £2,800–£4,500. Longer runs over 30m: add £200–£400 per additional 10m. Sleeved overhead run (alternative to trenching): £400–£900 depending on span.
