The outdoor sauna buying guide: five decisions before you spend
Buying an outdoor sauna is about more than choosing a shape. This guide covers five decisions that matter before you spend: format, size, heating, base and garden siting.
The outdoor sauna buying guide: five decisions before you spend
An outdoor sauna comes down to five decisions: the format, the size, what heats it, what it stands on, and where in the garden it can legally go.
Everything else is finish and colour.
This guide takes those decisions in the order they need to be made, because getting the first one wrong can make every decision after it more expensive.
1. Barrel or cabin: what are you actually choosing between?
Two formats dominate the outdoor sauna market: the barrel and the rectangular cabin.
A barrel sauna is a circular room made from solid timber staves. A cabin sauna is rectangular, giving more freedom over bench position, glazing, ceiling height and internal layout.
That is more than an aesthetic difference. You are choosing between different construction methods, internal geometry, footprints and ways of using the space.
| Barrel | Garden cabin | |
|---|---|---|
| Typical construction | Solid timber staves | Engineered timber construction |
| Room shape | Circular | Rectangular |
| Bench layout | Commonly facing benches | Along a wall, facing or L-shaped |
| Delivery | Often supplied as a kit | Panels or a built unit, depending on model |
The current Tarnfell range illustrates the difference. The Harvia Kuusi 180 Panorama is a 40mm solid spruce barrel with four seats and an 8kW heater included. The URSA uses 55mm cross-laminated timber walls and seats three to four.
But do not make the mistake of assuming that thicker walls automatically make one sauna better. Construction, insulation, glazing, room volume, heater sizing and how you intend to use the sauna all affect the result.
Choose the room first. Then understand the construction.
2. How big does it need to be?
Capacity is normally quoted in people. The heater, however, is sized using cubic metres.
That makes internal room volume more useful than the headline number of seats.
The current Tarnfell range includes everything from the two-person EMBA to the five- or six-person EDEN. The four-person Barrel Sauna 250 sits between those extremes.
Those figures do not tell you how comfortable the sauna will be inside, though.
Bench length, bench depth, ceiling height, heater position and the amount of glass can change the experience considerably.
The useful question is therefore not:
How many people can technically fit?
It is:
How many people will actually use it in a normal week, and how do I want them to sit?
If you regularly want to lie down, that changes the answer. If four people will normally use it together, buying for an occasional larger gathering may not make sense.
A larger room also means a larger heating requirement, so capacity affects more than the purchase price.
3. What heats it, and what does that ask of your house?
For an electric sauna, heater sizing starts with room volume.
As a starting point, manufacturers commonly work around 1kW of heater power per cubic metre, but the calculation changes when the sauna contains significant areas of glass or other poorly insulated surfaces.
Harvia's published guidance uses an adjustment of 1.2m³ for each square metre of glass or another non-insulated surface. The heater manufacturer's own calculation should always take precedence.
Our guide to sauna heater sizing works through the calculation in detail.
Glass changes the calculation
Take a sauna measuring:
2.0m × 1.8m × 2.0m = 7.2m³
Now give it a full glass front measuring 1.8m × 2.0m:
3.6m² of glass × 1.2 = 4.3m³
The effective calculated volume becomes approximately:
7.2 + 4.3 = 11.5m³
The cabin has not physically become bigger. But from the heater's point of view, the heating requirement has increased significantly.
That is why simply matching the physical room volume to a heater's kW rating can produce the wrong answer.
Heater size also affects your electrics
A 6kW heater at 230V draws approximately 26 amps. That is well beyond a normal 13A plug and socket, so an electric sauna heater requires an appropriate dedicated circuit.
The exact requirement depends on the heater. For example, the Harvia Vega specifications used in Tarnfell's current heater guide give:
- 6kW: 35A fuse
- 9kW: 50A fuse
The actual supply, cabling and protection requirements must be confirmed against the heater manufacturer's installation instructions and by your electrician.
This is why the electrical supply should be considered before you buy the sauna rather than after it arrives.
Electric or wood-fired?
Wood-fired heating changes the equation.
It removes the need for an electric heater circuit, but it replaces that requirement with a different set of considerations: the stove and flue, carbon monoxide protection, suitable fuel, smoke-control restrictions, chimney requirements, sweeping and maintenance.
Tarnfell's electric versus wood-fired comparison goes through the practical differences.
The right question is not:
Which heating system is better?
It is:
Which one fits the way I will actually use the sauna?
4. Will your garden take it?
This is where the buying decision becomes much more practical.
Before choosing a model, check:
- External dimensions.
- Overall height, including the roof.
- Distance to the nearest boundary.
- Width of the narrowest access point.
- Route from the road to the final position.
- Electrical route if the heater is electric.
- Base requirements.
- Space around the sauna for access and maintenance.
Planning permission
In England, garden saunas generally fall within the category of outbuildings. Many can be built under permitted development, but the conditions matter.
The rule that catches people most often is the two-metre boundary condition. Where an outbuilding is within two metres of a boundary, its maximum overall height is 2.5 metres.
Other permitted-development conditions cover matters including eaves height, overall building height, how much land around the original house can be covered, position relative to the principal elevation, listed buildings and designated land.
The rules differ in Wales, Scotland and Northern Ireland.
Read Tarnfell's planning permission guide before ordering, and check your circumstances with the relevant local authority where necessary.
The base matters more than it looks
A sauna needs a suitable base for the particular model.
That generally means a level, load-bearing and well-drained foundation, but the precise requirement depends on the construction. The Harvia Kuusi, for example, has a specific foundation and anchoring requirement; the URSA has a different base specification.
Do not assume that one base specification applies to every sauna.
And think about drainage and access at the same time.
Plan your space before you commit. It asks about the number of users, frequency of use, electrical supply, access width, base and smoke-control area because those questions can change the recommendation.
5. What will it cost to buy and run?
The current Tarnfell outdoor sauna range is deliberately short. Prices and availability change, so use the live outdoor sauna collection for the current position rather than treating a buying-guide table as a permanent catalogue.
At the time of writing, the range spans roughly £6,240 to £23,980, with different construction, capacity, heater and installation arrangements across the models.
Do not compare the headline sauna price alone. Depending on the model, you may also need to account for:
- base preparation;
- electrical installation;
- delivery;
- installation or assembly;
- access requirements;
- and additional equipment required by the heating system.
What does an outdoor sauna cost to run?
Running cost is more complicated than multiplying the heater's kW rating by your electricity price.
A thermostatically controlled electric heater does not necessarily draw its full rated power throughout an entire session.
Harvia gives an example of approximately 7–9kWh for a two-hour session using a 6kW family sauna, while actual consumption varies with construction, starting temperature, outdoor conditions, target temperature, glazing and usage.
At the Ofgem electricity price-cap rate used by Tarnfell for July to September 2026, that works out at roughly £1.83–£2.35 per two-hour session.
At three sessions a week, that is approximately £285–£367 per year.
Those are illustrations, not promises.
Actual consumption depends on:
- cabin construction;
- insulation;
- glazing;
- outside temperature;
- starting temperature;
- target temperature;
- heater sizing;
- session length; and
- how often the door is opened.
So do not look at 9kW and assume it will cost exactly 50% more to run than 6kW. The real picture is more complicated.
The five decisions, in order
- Format: barrel or cabin?
- Size: how many people will actually use it, and what internal space do you want?
- Heating: electric or wood-fired, and what does that require from the property?
- Base and access: can the sauna physically reach the garden, and can the site provide the right base?
- Siting: will the proposed position work under the relevant planning and access requirements?
Only after those five decisions should you worry about colours, lighting and finishes.
Three measurements to take before you buy
1. The available footprint
Measure the actual area where the sauna will stand, then check the manufacturer's external dimensions.
2. The route
Measure the narrowest point from where the delivery vehicle can stop to the proposed position.
Do not just measure the garden gate. Check gates, walls, steps, corners, overhead obstructions, slopes and the final carrying route.
3. The boundary distance
Measure from the proposed sauna position to the nearest boundary, then check the sauna's overall height.
The bottom line
Buying an outdoor sauna is not really a choice between attractive timber buildings.
It is a decision about room geometry, heating, construction, electricity, access, planning and ongoing ownership.
Get those decisions right and the product comparison becomes much easier.
Get them wrong and the most attractive sauna on the website can be the wrong sauna for your garden.
Start with the measurements. Then choose the format. Then size the room. Then choose the heating system. Then check the site.
Buy once. Choose well.
