Holl's Onyxia Cube: 4 Person Outdoor Sauna, Glazed Front
One configuration. The 8kW stove and its stones are in the price; the base and the circuit are yours to arrange.
£10,995.00
Harvia heater inside
- 40mm Finnish ThermoWood walls
- Harvia Vega 8kW stove and stones included
- 35A fuse per Harvia, 34.8A at 230V
- Cabin
- 40mm Finnish ThermoWood, 2,100mm wide, 1,600mm deep and 2,200mm tall, in waterproof black
- Inside
- Raw pine benches at two heights for four, with white LED strips behind the seat backs
- Heater
- A Harvia Vega 8kW stove with its stones
- Front
- A glazed front panel and an 8mm black smoked tempered glass door
- Also supplied
- A floor protection mat
- Electrics
- A dedicated 35A circuit in armoured cable, wired and certified by an electrician
- Delivery
- One pallet, to the kerb
A black cube 2.1 metres wide, 1.6 deep and 2.2 tall, built from 40mm Finnish ThermoWood with a glazed front and an 8mm smoked glass door. Four people on two bench heights. The 8kW stove and its stones are in the price.
Delivery & installation
Delivery is included in the price, kerbside to mainland Britain: England, Wales and mainland Scotland. The Scottish Highlands, islands and any address a lorry cannot reach are quoted in writing before dispatch, and if the figure does not suit you, cancel for a full refund. Northern Ireland: write to us before ordering. Nothing is added at checkout.
Delivery to a mainland UK address is in the price. The cabin arrives on a pallet, set down at the kerb or the nearest firm, level spot the driver's pallet truck can reach; a pallet truck does not cross gravel, grass or a step. From there the parts move by hand, and this is a three to four person job rather than a two person one. We confirm the delivery arrangement for your address in writing when we process your order.
Offshore addresses, the Scottish Highlands and islands, and any address a lorry cannot reach are quoted separately before you order.
Installation is not in the price. It can be arranged for your site where you would rather not build it yourself, and the cost is confirmed before anything is delivered.
Warranty
Four years on the timber and two years on the electronics, the terms Holl's publish for this range.
The UK distributor administers the warranty and raises claims with the maker where needed. The cabin is sold for home use: commercial or very frequent use can affect cover.
Claims come to us. One email, and we deal with the distributor and the maker on your behalf rather than handing you a phone number.
Returns
Until the order is placed with the supplier you can cancel and be refunded in full, with no reason needed. Your statutory cancellation rights are set out in full in our returns policy.
Check the pallet on the day it arrives, count the parts against the list in the manual, and tell us about anything damaged or missing within 48 hours with photographs. Anything that arrives damaged or faulty is ours to put right. Do not book an electrician or an installer for a fixed date until the delivery has arrived and been checked.
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About this product
A black cube 210cm wide, 160cm deep and 220cm tall, built from 40mm Finnish ThermoWood and finished in a waterproof black paint, with a glazed front and an 8mm black smoked glass door. Four people on two bench heights, with white LED strips set behind the seat backs rather than shining at them.
The price includes the heater. A Harvia Vega 8kW stove and its stones ship with the cabin.
40mm is the number worth noticing. Most garden cabins in this price range are built from thinner stock, and wall thickness is what decides how long a cabin holds temperature once the stove cycles off. A floor protection mat is supplied.
At 230V an 8kW element draws 34.8A, which is our arithmetic, and Harvia publish a 35A fuse for this heater. It runs on a dedicated circuit in steel wire armoured cable on a 30mA RCD, with the protective device sized and certified by a qualified electrician. That is the same class of load as an electric vehicle charger, so it is worth an electrician's assessment before you order.
Four year warranty on the timber, two years on the electrics, administered through the UK distributor.
- Secure paymentProcessed by Shopify Payments over an encrypted connection.
- Your details stay privateCard details are never stored on a Tarnfell system.
- Payment on confirmationWe take payment only once your order is confirmed.
- 14 day cancellation rightsCancel within 14 days of delivery, under the Consumer Contracts Regulations 2013.
Who this is actually for.
Every product here is described with the honest opposite beside it: who it suits, and who should be looking at something else. If none of the descriptions below fits you, the right answer is elsewhere in the range, or elsewhere entirely, and we would rather say so.
40mm of Finnish ThermoWood is the number that matters here. Wall thickness is what decides how long a cabin holds temperature once the stove cycles off, and 40mm is at the heavier end of what garden cabins are built from. The black finish, the glazed front and the smoked glass door are why people choose it; the wall is why it works.
Worth confirmingyou want the cheapest four person cabin. This is not it, and the extra buys timber thickness and finish rather than seats. If budget is the binding constraint, a barrel seats more people for less.
8kW at 230V is 34.8 amps on a dedicated 35 amp circuit in armoured cable. That is the same class of load as an electric vehicle charger, and whether your supply has the headroom is a calculation an electrician does on site.
Worth confirmingnobody has looked at your consumer unit yet. Find out first. It is a short question for an electrician and it decides whether this cabin is possible at all.
The product that gets used is the one worth recommending, and it is not always the largest one.
The measurements that decide it.
Read this before anything else. Things this size are almost never sent back because they were the wrong product. They are sent back because the pallet could not be set down, the parts would not reach the spot, or the ground under them was never going to hold them.
A cube uses a narrow plot better than a barrel, because nothing is wasted at the corners and it can sit closer to a boundary while staying under 2.5 metres. The 40mm walls make the panels heavy for their size, so plan the carrying route rather than the parking.
From order to first use.
Here is each stage, and who does what. Delivery is estimated at 10 to 14 working days from order. If you would rather have stock, the date or anything else settled first, email hello@tarnfell.com before you order and we will confirm it in writing.
- Week 0
Order and site check
Base, access and your electrical supply are agreed before the order is placed with the distributor. You arrange the base and the circuit. There is nothing to configure on the cabin itself.
- Before it arrives
The base and the circuit
A flat, hard, level base at least 2.1 by 1.6 metres, and an electrician runs a dedicated 35A circuit to it in armoured cable and leaves it ready to terminate. Outdoor circuits and rooms containing a sauna heater are notifiable work in England, so this is not a job for a competent friend.
- Delivery day
One pallet, set down at the kerb
The cabin arrives on a pallet at the kerb or the nearest firm, level spot the pallet truck can reach. Count the parts against the list that day and photograph anything damaged. Moving the parts to the base is a three to four person job.
- Build
Base frame, walls, glazing, benches
The frame goes down on the base, then the 40mm wall panels, the glazed front and the smoked glass door, then the two bench heights and the floor mat, working from the maker's instructions. Three to four people. If you would rather hand it over, installation can be arranged for your address once you have ordered.
- Before first use
The electrician returns
The stove is fitted, connected and the circuit tested and certified. You should be handed a certificate. Then the stones go in, loosely, and the first heat cycle burns off the factory smell with the door open.
What owning one actually involves.
The handful of things that decide whether this is still in use in year three.
A flat, load bearing slab or paved base at least 2.1 by 1.6 metres, laid before the pallet arrives. A base that settles takes a glazed door out of square faster than a solid one, and on this cabin the door is 8mm glass, so it is worth getting level right rather than nearly right.
8kW at 230V is 34.8 amps on a dedicated 35 amp circuit in armoured cable, protected by a 30mA RCD and certified by an electrician before first use. Worth knowing before you order: that is the same class of addition as an electric vehicle charger, and not every house supply has the headroom.
ThermoWood is pine cooked rather than treated, which drives out the sugars that feed rot and leaves timber that moves less with the weather. The outside carries a waterproof black paint, so unlike a bare cabin this one is maintained by keeping the coating sound rather than by letting it silver.
A black cabin in July absorbs noticeably more solar gain than a pale one, which shortens warm up in summer and makes the interior warm even unheated on a bright day. It also shows a scuff more plainly than bare timber does, and the coating is what you are maintaining rather than the wood.
The running costs, with the working shown.
The purchase price is public; the running cost rarely is. Ours are published with the assumptions stated, so you can check the arithmetic and change it for your own tariff.
about £3.13
Heat up: 8kW for an hour is 8 kWh, for a cabin of about 7.4 cubic metres with 40mm walls. Holding: 1 hour at roughly 50% duty is 4 kWh. Total 12 kWh at 26.11p is about £3.13. The thicker wall is the reason this sits below a thinner cabin of the same size.
about £326
104 sessions at £3.13. A two seat outdoor cabin on a 4.5kW stove comes to about £152 on the same sums.
a few pounds, every few years
Sauna stones crack and crumble with use, are restacked yearly and replaced when they do. They ship with the stove and they are the only consumable part.
Assumptions: electricity at the Ofgem price cap unit rate of 26.11p per kWh, 1 July to 30 September 2026, including VAT. Figures are calculated from the stove's rated output of 8kW, not measured on an installed cabin. One hour to heat up is our working figure.
What you have to do, and what the rules require.
Everything you have to do, and how often.
| Job | How often | Who |
|---|---|---|
| Install and certify the stove circuit 8kW draws 34.8A at 230V, so a dedicated 35A circuit in steel wire armoured cable on a 30mA RCD. Outdoor circuits and rooms containing a sauna heater are notifiable work in England. | Once, before first use | A qualified electrician |
| Lay the base A flat, load bearing slab or paved base at least 2.1 by 1.6 metres, able to carry the cabin plus the people in it. | Once, before delivery | You, or a groundworker |
| Build the cabin Frame, 40mm wall panels, glazed front, smoked glass door, two bench heights and the floor mat, following the maker's instructions. Three to four people. | Once, over a weekend | You and two or three others, or an installer |
| Restack and inspect the stones Lift them out, discard any that are cracked or crumbling, and restack loosely so air moves between them. Tightly packed or broken stones make the elements work harder. | Yearly | You |
| Dry it out after wet weather Heat the cabin after rain and once a month if it is standing unused, to keep surface mould off the timber. Air it after every session. | After rain; monthly when idle | You |
| Oil or stain the outside, if you want the colour kept This cabin is painted rather than bare, so the job is keeping the coating sound rather than letting it silver. Check it each spring and touch in any chip before water gets behind it. | Every year or two, if you want the colour kept | You, an afternoon |
| Check the door and glass seals A cabin that heats and cools moves, and a base that settles takes the door out of square. Check the door closes cleanly and the glazing beads are sound at the start of each season. | Yearly | You |
Where this comes from.
Regulation and guidance, linked so you can read it yourself. None of it is our opinion.
- Outdoor circuits and rooms containing a sauna heater are notifiable electrical workApproved Document P, 2013 edition
- Outbuildings within 2 metres of a boundary are limited to 2.5 metres in height under permitted developmentPlanning Portal, outbuildings
- A heater needs about 1 kW for each cubic metre of sauna volume, and a square metre of glass or stone adds about 1.2 cubic metres to that requirementHarvia, how to select the correct heater power
- A 35 amp addition sits in the same load class as an electric vehicle charger or a hot tub, which the distribution network operator names as reasons a house may need a larger main fuseUK Power Networks, what do you need to upgrade
- Electricity unit rate of 26.11p per kWh, 1 July to 30 September 2026Ofgem energy price cap
Every figure the makers publish.
- Warranty
- 4 year wood, 2 year electrical warranty
- Lead time
- Estimated 10 to 14 working days from order. We confirm stock and your delivery date in writing when we process your order.
Cabin figures are from Holl's listing for the Onyxia Cube and the UK distributor's product page. Where a figure is ours rather than the maker's, it says so.
What arrives, and what does not.
- The cabin: the 40mm ThermoWood structure in waterproof black, the glazed front, the smoked glass door, the raw pine benches at two heights and every fixing
- The Harvia Vega 8kW stove and its stones
- White LED strips behind the seat backs, a floor protection mat, and the maker's assembly instructions
- Delivery on one pallet to the kerb at a mainland UK address
- Four years of the maker's warranty on the timber and two on the electronics
- One point of contact for the order, the delivery and anything that goes wrong: us
- The base. A flat, load bearing slab or paved area at least 2.1 by 1.6 metres
- The electrical work. A dedicated 35A circuit, run, terminated and certified by an electrician
- Assembly, unless you ask us to arrange it
- Moving the pallet beyond where the pallet truck stops. From there the parts are carried by hand
- Touch up paint for the black coating
- Delivery to offshore and remote postcodes, which is quoted before you order
Reasons to buy something else.
Delivery is by lorry to the kerb. A shared private road, steps to the kerb, a gravel drive or a double parked frontage all need solving before you order, not on the day. From wherever the pallet stops, every part moves by hand, and 40mm panels are heavy for their size.
The stove will not run from a socket. 34.8 amps on its own circuit is the same class of addition as an electric vehicle charger, and whether your house can take it is a calculation an electrician does on site rather than something anyone can promise from a web page.
This cabin seats four and costs more than barrels that seat six. What the money buys is 40mm walls, a painted weatherproof shell and a glazed front, not capacity. If seats per pound is the measure, a barrel wins and we will say so.
A 40mm wall holds heat better than a thin one, but it is still a single skin outdoor cabin rather than an insulated building. It heats and cools faster than a room in your house, and in January the 8kW stove is working. If year round evening use at the lowest running cost is the aim, a fully insulated cabin costs more for exactly that reason.
If any of these apply, say so and we will point you at what does suit you, including when that is a cheaper product or one we do not sell.
What we know, and how we know it.
Where every figure on this page comes from, with the source beside it.
- Dimensions, timber, glazing and what is supplied
-
Verified
Taken from Holl's listing for the Onyxia Cube and the UK distributor's product page, which agree on 210 x 160 x 220cm, four seated, 40mm Finnish ThermoWood, the waterproof black finish, raw pine benches at two heights, the 8mm black smoked glass door, the white LED strips behind the seat backs and the floor protection mat.
- Electrical supply
-
Verified
8,000 watts at 230 volts is 34.8 amps, which is our arithmetic rather than a claim. Harvia publish a 35 amp fuse for this heater, and the final protective device is the installing electrician's calculation on site. Armoured cable and a 30mA RCD are what an outdoor circuit requires under BS 7671. The cable size is the installer's calculation on site and this page does not state one.
- Weight, pallet and assembly
Three to four people is the handling figure the distributor gives. The distributor's guidance for this range is 10 to 14 working days, which is the estimate on this page, and we confirm your date in writing when we process your order.
- Warranty administration
Four years on the timber and two on the electronics are the maker's published terms. Claims run through the UK distributor, which raises them with the maker where needed, and you deal only with us. The cabin is sold for home use: commercial or very frequent use can affect cover.
The three closest to it.
Holl's Onyxia Barrel: 6 Person Outdoor Sauna
£10,995.00- 6 people, 225 × 240 × 226cm
- 40mm ThermoWood
- 8kW Harvia Vega stove included
Holl's Hestia Lounge 210: 4 to 6 Person Outdoor Sauna
£8,990.00- 4 to 6 people, 205 × 210 × 220cm
- Covered terrace
- 6kW stove included
Holl's Orphée Lounge 240: 5 to 6 Person Outdoor Sauna
£8,990.00- 5 to 6 people, 205 × 240 × 215cm
- Barrel with covered terrace
- 9kW Holl's Vulkano stove included
Weighing this against another sauna? Let us do the homework.
Choosing between two saunas takes evenings of research, so let us carry it. Name the other one you are considering and we will send a straight comparison, including where the other one wins.