Wiring up a garden office from scratch — where do I start with the consumer unit?

Finally getting round to building a proper insulated garden office at the bottom of the garden and I want to do this right from the off. The plan is to run a 6mm² SWA cable from the house (about 18 metres), feed into a small consumer unit in the office, and have enough capacity for a desk setup, a couple of monitors, a small fan heater for winter, and decent LED lighting throughout. Nothing mental, but I want it safe and future-proof.

My main head-scratcher is the consumer unit setup. I've been looking at a 6-way Wylex unit with a 40A main switch, with a 32A Type B breaker for the ring (or should that be a radial at this distance/load?), a 6A for lighting, and an RCD upstream of the lot. Someone mentioned I should be looking at a split-load board instead so the lighting doesn't go dark if the socket RCD trips — which makes sense to me, but I'd love to hear how others have actually done it.

Has anyone gone down the route of adding a small battery/solar backup into the mix as well? I'm wondering whether it's worth roughing in a second incomer position now while the walls are still open, even if I don't fit the inverter until later. Feels like the kind of thing I'd kick myself for not doing when I had the chance.

@SparkySailor nice project! Few things worth thinking about before you spec the consumer unit:

Earthing arrangement matters first — SWA armour can serve as your earth conductor back to the house TN-C-S/TN-S system, but you'll want to confirm your DNO supply type and whether the armour cross-section is adequate. Many folks add a separate earth conductor inside the SWA just to be safe.

Isolation at both ends — you'll need a means of isolation at the house and at the garden office. A small 4-way consumer unit with a 63A main switch is perfectly sensible. Wylex and Crabtree units are decent value.

One thing I'd genuinely recommend: if there's any chance of adding solar/battery later (I went down that rabbit hole with a Victron Multiplus and Fogstar Drift cells), leave yourself a spare way in the CU from day one. Future-you will be grateful.

@SparkySailor done almost exactly this for my own garden office — 6mm² SWA, Wylex consumer unit with a 32A MCB feed, RCD protection the lot.

One thing nobody mentions until it's too late: treat the office as a separate building for earthing purposes and fit a local earth electrode rather than relying on the house TN-C-S — DNOs get very twitchy about extending PME to outbuildings, and for good reason.

Also budget a proper isolator at both ends of that SWA run — future you will be extremely grateful when you're fault-finding in the rain.

Basically: do the earthing properly first, then everything else is just shopping.

@SparkySailor one thing worth adding to what @WheresMeWires and @ExPostie86 have mentioned — don't underestimate your future load requirements when sizing the consumer unit. It's tempting to fit a small 4-way unit now, but going slightly bigger costs pennies in comparison to rewiring later. I'd suggest a 6-way minimum.

Also, get your SWA properly glanded at both ends — it's an earth in itself but only if those glands are making solid contact. I've seen folk skimp on the glands and then wonder why their RCD is tripping randomly.

One more thing — notify your local authority under Part P before you start. Outbuildings aren't exempt.

Great project @SparkySailor! One thing I'd add that nobody's mentioned yet — think carefully about your earthing electrode if you're going TT out there. A lot of people just bang in a single earth rod and call it a day, but getting a decent resistance to earth can be trickier than expected depending on your soil type. Dry sandy soil especially can give you grief. You might need multiple rods or a longer one than you'd think. Worth doing a proper earth loop impedance test once it's in rather than just assuming it's fine. Also, if you're planning any IT equipment in the office, a good earth is doubly important for both safety and keeping things running cleanly. Don't skimp on that bit! 🙂

Great advice from everyone so far. One thing I'd add — given you're running 6mm² SWA, make sure you properly terminate the armour at both ends using appropriate SWA glands. The armour itself can serve as your earth continuity back to the house, but only if those glands make solid contact. A lot of people bodge this bit and end up with a dodgy earth path.

Also worth thinking about your cable route — if it's going underground, you'll want it buried at least 500mm deep (deeper under a driveway) and ideally run through conduit or with cable covers plus a warning tape above it. Keep a sketch of the route too, you'll thank yourself in a few years when someone's digging!

@Dizzy75 makes a good point about earthing electrodes — definitely worth considering depending on your setup.

Great thread this, lots of solid advice already from @WheresMeWires, @Dizzy75 and others.

One thing I'd flag that hasn't been touched on yet — don't forget your garden office will need its own RCD protection at the outbuilding end, not just relying on whatever's protecting the circuit back at the house. A small consumer unit with a main switch and a couple of RCBOs is the way I'd go rather than a single RCD covering everything. Means if your kettle trips out it won't kill your computer at the same time!

Also worth checking whether your DNO needs notifying about the additional load before you even start. Most domestic installs are fine but worth a quick call. And obviously the whole job needs notifying to Building Control under Part P. Don't skip that bit — it'll cause headaches when you come to sell the house.

Really useful thread this. Quick question for anyone who's done similar — are you planning to have any battery backup in there @SparkySailor? I've got a small Victron setup on my narrowboat and I'm wondering whether it's worth incorporating something like that into a garden office from the start, rather than retrofitting later. Feels like the consumer unit choice and overall design might differ slightly if you're planning a hybrid setup eventually. Has anyone here gone down that route with a grid-tied inverter/charger in a garden office?

Great project @SparkySailor! One thing worth mentioning that I haven't seen covered yet — don't forget to register the installation with your local building control, or use a Part P registered electrician to sign it off. A lot of people skip this step on outbuildings and it can cause real headaches when you come to sell the house. Insurers can also be funny about it if something goes wrong. Totally doable as a DIY job if you're competent, but that certification piece is non-negotiable in my experience. Worth budgeting for an EICR at completion too.

Great project @SparkySailor! On the consumer unit itself — get a dual RCD or RCBO board rather than the old-school split load type. Future you will thank present you massively.

Also, totally jumping on @Emma1996's question — I've got a Victron SmartShunt and a couple of Fogstar Drift cells

@SparkySailor don't overlook earthing — your SWA armour can serve as your CPC but only if the installer actually tests it properly, and "properly" is doing a lot of heavy lifting there given how many garden office installs I've seen bodged with a bit of wishful thinking and a prayer to the BS7671 gods.