Outdoor Base Requirements UK — Saunas, Swim Spas, Hot Tubs & Pergolas
Outdoor Base Requirements: Saunas, Swim Spas, Hot Tubs & Pergolas
Concrete pad thickness, paving slabs, decking, ground-screw alternatives — everything you need to know before your delivery date so the base is ready and the install goes smoothly.
Quick answer
Every Nuovo Luxury outdoor product needs a level, structurally adequate base in place before delivery. Saunas need a flat hard-standing rated for ~600–1,200 kg total (4-inch / 100mm reinforced concrete pad is the gold standard). Swim spas and hot tubs need a properly engineered concrete base — typically 6 inches (150mm) of reinforced concrete on prepared sub-base, rated for the filled weight which can exceed 4,000–8,000 kg. Pergolas need anchored footings under each post — concrete pads, pier footings, or chemical anchors into existing concrete depending on the model. Always confirm exact requirements against the product's installation manual before pouring.
Why the base matters more than buyers expect
The single most common cause of post-installation problems with outdoor saunas, swim spas, and pergolas is a base that wasn't quite right. Symptoms range from cosmetic (doors that stop closing flush after a year because the unit has settled) to serious (water ingress, structural cracking, voided warranty). All of these are entirely preventable with the right base preparation up-front.
The good news: getting it right isn't complicated. The base needs to be flat, level, and capable of supporting the filled weight of whatever sits on it. The skill is matching the specification to the product, the ground conditions, and how the unit gets onto it.
Sauna base requirements
A pre-assembled outdoor cabin sauna typically weighs 600–1,500 kg dry, depending on size. That's a lot of point load over a relatively small footprint, and once heat cycling, moisture, and weather are factored in, the base needs to hold the unit level for decades.
Recommended sauna base options
| Base type | Specification | Best for |
|---|---|---|
| Reinforced concrete pad | 100mm (4 inch) thick, C25 mix, A142 mesh reinforcement, on 100mm compacted MOT Type 1 sub-base | The gold standard. Suits any sauna, any ground type, lasts indefinitely. |
| Paving slabs on prepared sub-base | 50mm flagstones on 50mm sharp sand on 100mm compacted MOT Type 1 | Smaller saunas (2–4 person) on firm, well-drained ground. Cheaper than concrete. |
| Existing patio (modern, structural) | Verify level and that flags are bedded on mortar, not loose-laid on sand | Where a recent, properly laid patio already exists in the right spot. |
| Decking | Heavy structural deck on appropriately spaced joists (~400mm centres), capable of point loads | Visually beautiful, common in landscape-led gardens. Build to engineering spec. |
| Ground screws / helical piles | 4–6 screws, length determined by ground conditions, with a steel frame above | Sloping sites, poor ground, listed property gardens where digging is undesirable. |
Sauna base sizing
The base should match the sauna's footprint exactly, or extend a small step beyond it (typically 50–100mm). It should not be significantly larger — water sitting on exposed base around the sauna can cause splash-back staining and accelerated weathering of the lower walls. If you want a wider patio surround for cooling space, lay it as a separate step lower than the sauna base.
Drainage around saunas
Saunas are dry inside but wet outside — rain, snow, splash-back from showering, and condensation all need somewhere to go. The base should either:
- Slope very gently away from the sauna (1–2% / approximately 10–20mm per metre) toward a drained area, or
- Sit on free-draining ground (gravel, well-drained soil) where water can dissipate naturally
Don't pour the base in a hollow where water collects. If your only suitable spot is low-lying, add a French drain or soakaway around it before pouring.
Swim spa & hot tub base requirements
This is where bases really matter. A filled swim spa or hot tub is heavy — often heavier than two filled cars sitting on top of each other in a 2 × 4 metre footprint.
Filled weight expectations
| Product type | Typical dry weight | Typical filled weight | Footprint |
|---|---|---|---|
| Standard hot tub (4–6 person) | 250–400 kg | 1,800–2,800 kg | ~2.1 × 2.1m |
| Premium hot tub (6–8 person) | 400–600 kg | 2,800–4,000 kg | ~2.3 × 2.3m |
| Ice bath (e.g. Chill Tubs) | ~140–200 kg | 400–600 kg | ~1.6 × 0.8m |
| Swim spa (compact) | 1,200–1,600 kg | 5,000–6,500 kg | ~4.5 × 2.3m |
| Swim spa (large / dual zone) | 1,600–2,200 kg | 7,000–9,000+ kg | ~5.5 × 2.3m |
Concrete base specification for swim spas & hot tubs
- Hot tubs (under 3,000 kg filled): 100mm (4 inch) reinforced concrete on 100mm compacted MOT Type 1 sub-base. C25 mix minimum. A142 mesh reinforcement.
- Larger hot tubs & small swim spas (3,000–6,000 kg filled): 150mm (6 inch) reinforced concrete on 150mm compacted MOT Type 1 sub-base. C25 or C30 mix. A252 mesh reinforcement.
- Large swim spas (6,000–9,000+ kg filled): 200mm (8 inch) reinforced concrete on 200mm compacted MOT Type 1 sub-base. C30 mix. A393 mesh reinforcement, or consult a structural engineer for sites with poor sub-soil.
- Ice baths (under 600 kg filled): Existing patio or decking is usually fine, provided it's level and structurally sound.
Levelness for water-filled units
Swim spas and hot tubs are extremely sensitive to levelness — far more than saunas. Water finds the lowest point, which means an out-of-level base causes uneven water depth, jets working unevenly, and stress on the shell. Target tolerance is under 10mm across the entire base in any direction. Don't pour the base by eye — use a laser level or a long straight edge with a spirit level.
Service access
Leave 600mm of clear access around at least one full side of the hot tub or swim spa for service. Equipment compartments (pumps, heaters, filtration) need to be reachable for maintenance and the occasional repair. The base itself doesn't need to extend into this access zone — but the surrounding paving or surface should support a kneeling engineer with tools.
Pergola pad & footing requirements
Pergolas are different from saunas and swim spas — they're wind-loaded structures rather than weight-loaded. The base needs to resist uplift (wind trying to lift the structure) and overturning (wind trying to topple it), not just support the dead weight.
Footing options for pergolas
| Footing type | Specification | Best for |
|---|---|---|
| Chemical anchors into existing concrete | M12–M16 stainless anchors, set into modern, structurally sound concrete or a high-grade patio | Existing patios where the concrete is thick enough (usually 100mm+) and well laid |
| Individual concrete pad footings | 400 × 400 × 400mm minimum per post, reinforced, depth into ground depending on soil | Lawns, gravel gardens, or any site without existing hard standing — the most common spec |
| Continuous reinforced concrete pad | 100mm reinforced slab covering the full pergola footprint | Where the pergola sits over a new patio or full hard-landscaping scheme |
| Ground screws / helical piles | One per post, sized to the pergola's wind load and your ground conditions | Where digging is impossible or undesirable — gardens with services, listed sites, soft ground |
PergoEnvision-specific footing notes
Our PergoEnvision glass garden rooms are large, structural enclosures with significant wind loading from the glass walls and louvred roof. Each model ships with a specific footing schedule in the installation manual. As general planning guidance:
- Posts must be anchored into structural concrete — either an existing slab rated for the load, or new individual pad footings minimum 400 × 400 × 600mm deep
- Anchor points must be set to exact survey dimensions before any concrete cures — your installer will normally template these on site
- Sites with high exposure (coastal, hilltops, open valleys) may need deeper footings; we'll flag this at the survey stage
Concrete specifications explained
If your builder or landscaper asks "what mix?" — here's what the codes mean:
- C25 (25N/mm²): Standard residential concrete. Suitable for saunas, hot tubs, and most pergola footings.
- C30 (30N/mm²): Slightly higher strength. Recommended for large swim spas, areas with light vehicle access, or sites with marginal sub-soil.
- C35 / C40: Structural concrete. Only required for very heavy commercial swim spas or where a structural engineer specifies it.
Reinforcement codes:
- A142 mesh: 6mm wires at 200mm spacing — light reinforcement, suits residential sauna and hot tub bases
- A252 mesh: 8mm wires at 200mm spacing — medium reinforcement, suits larger swim spas
- A393 mesh: 10mm wires at 200mm spacing — heavy reinforcement, large swim spas or marginal ground
Mesh sits on chairs in the middle of the slab thickness, with concrete above and below. Your builder will know exactly what this means — these terms exist so you can specify the job correctly when getting quotes.
Alternatives to a concrete pad
Paving slabs
Suitable for saunas up to ~1,000 kg. Use 50mm-thick concrete or natural stone flags, fully bedded on mortar (not loose-laid on sand), on a 100mm compacted MOT Type 1 sub-base. Not suitable for swim spas or large hot tubs.
Decking
Visually striking, common in landscape-led gardens. Suitable for saunas if the deck is engineered for the point load — typically requires:
- Joists at 400mm centres (not the standard 600mm)
- Doubled-up joists directly under heavy load points
- Posts on concrete pads at appropriate intervals
- Boards rated for the load span
Most pre-existing residential decking is built for foot traffic, not 1,000+ kg point loads. If you're decking specifically to receive a sauna, design it that way from the start. Decking under hot tubs and swim spas is rarely advisable — the loads exceed normal deck specifications by 5–10x.
Ground screws & helical piles
An increasingly popular alternative to concrete — steel screws are drilled into the ground at fixed positions, then a steel sub-frame is built on top to receive the sauna or structure. Advantages:
- No concrete pour, no curing time, no aggregate deliveries
- Reversible — can be unscrewed and the site restored if the sauna is ever moved
- Suits sloping sites, sites with poor drainage, or listed-property gardens
- Installed in a day rather than two weeks (pour + cure)
Disadvantages: more expensive than a small concrete pad for simple flat sites; needs specialist installers; ground type must be suitable.
Ground preparation & drainage
Whatever base type you choose, the ground beneath it matters. Steps in order:
- Excavate to the depth required by your base type — typically 200–300mm for a concrete pad (sub-base + slab thickness)
- Compact the sub-grade (the natural ground at the bottom of the excavation) — this prevents long-term settlement
- Lay and compact MOT Type 1 hardcore in 50mm layers, watered and rolled — this is the structural sub-base
- Lay damp-proof membrane for concrete pads where moisture rise is a concern
- Position reinforcement mesh on plastic chairs to ensure it sits in the middle of the slab thickness
- Pour concrete and finish to the required level and surface
- Cure for at least 7 days before any load is applied — longer in cold weather
If your ground is unusual — recently disturbed, very soft, sloping, has services buried in it, or is over a former pond — get a builder or structural engineer to look at it before you commit.
Planning permission for the base
In most cases in the UK, the base itself doesn't require planning permission — it's the structure on top that's regulated. Exceptions:
- Conservation areas and listed properties — always check with your local planning authority before any hard landscaping
- Front gardens — paving over front gardens above a certain size requires planning permission unless using permeable surfaces
- Sites close to neighbouring property boundaries — building regulations apply for certain structures
The pergola or sauna itself may have planning permission considerations (height, distance from boundary, conservation status) — check our individual product guides or your local council.
Cost & timeline
Costs vary regionally — these are UK planning-guide figures for 2025/2026:
| Base | Typical cost | Timeline (excl. cure) |
|---|---|---|
| 4 × 3m sauna pad (100mm reinforced concrete) | £800–£1,800 | 2–3 days work |
| 5 × 3m swim spa pad (150mm reinforced) | £1,800–£3,500 | 3–5 days work |
| Pergola pad footings (set of 6) | £600–£1,500 | 1–2 days work |
| Ground screws (4–6 screws for sauna frame) | £1,200–£2,500 | 1 day |
| Continuous reinforced slab for full PergoEnvision | £3,500–£7,000 | 4–7 days |
Concrete curing time matters: don't book delivery until your pad has cured for at least 7 days (14 days in winter). Light foot traffic from day 3, but full structural strength takes 28 days. Most installations are fine on day 7–10 for placement.
Related guides
- Pre-Assembled Sauna Delivery & Installation Guide — HIAB, crane and access requirements
- Electrical Requirements for Outdoor Saunas — circuit, MCB, Part P
- Preparing Your Garden for a Sauna — the full pre-delivery checklist
- Sauna Heater Sizing Guide — pick the right kW for your cabin
- PergoEnvision Glass Garden Rooms — installation overview
Frequently asked questions
How thick should a concrete pad for an outdoor sauna be?
For a residential outdoor sauna, 100mm (4 inches) of reinforced concrete on a 100mm compacted MOT Type 1 sub-base is the standard specification. Use a C25 concrete mix and A142 mesh reinforcement positioned in the middle of the slab. This suits any sauna up to ~1,500 kg dry weight.
How thick should a concrete base be for a swim spa?
For a compact swim spa with a filled weight of 5,000–6,500 kg, the typical specification is 150mm (6 inches) of reinforced concrete on a 150mm compacted MOT Type 1 sub-base, using a C25 or C30 mix with A252 mesh reinforcement. Larger swim spas (7,000+ kg filled) usually need 200mm (8 inches). Always confirm against your specific model's installation manual.
Can I put a sauna or hot tub on paving slabs?
Saunas up to about 1,000 kg can sit on 50mm-thick paving flags fully bedded on mortar on a properly prepared sub-base. Hot tubs and swim spas usually cannot — the point loads exceed standard paving capacity. Stick to engineered concrete or consult a structural engineer if you want to use paving for a heavier product.
Can I put a hot tub or swim spa on decking?
Decking under a filled hot tub or swim spa is rarely advisable. A 6-person hot tub weighs 2,800 kg filled — about 10 times the load a standard residential deck is designed for. Saunas on properly engineered decking with closely spaced joists are fine, but for water-filled units, build a concrete or paved base.
How level does the base need to be?
For saunas, 5mm tolerance across 3 metres is fine. For hot tubs and swim spas, tighter — aim for under 10mm across the entire base in any direction. Water-filled units are far less forgiving than dry units. Use a laser level or a long straight edge with a spirit level, not visual judgment.
How long does concrete need to cure before delivery?
Minimum 7 days, ideally 14 days for residential bases supporting heavy loads. In cold weather (below 5°C), curing time doubles. Full structural strength takes 28 days, but most installations are fine on day 7–10 for placement. Don't book delivery for the week of the pour — leave a buffer.
Do I need planning permission for the base?
In most cases, no — the base itself is rarely planning-relevant. Exceptions: conservation areas, listed properties, front gardens above a certain size, or sites close to boundaries. Check with your local planning authority if any of these apply. The structure on top of the base may have its own planning considerations.
What's MOT Type 1 and why does the spec keep mentioning it?
MOT Type 1 is a graded crushed stone aggregate (specified by the UK Department for Transport, hence the name) widely used as a structural sub-base under concrete, paving, and roads. It compacts to a stable, free-draining layer that prevents settlement. Most builders' merchants stock it as standard.
Can I use ground screws instead of a concrete pad?
Yes, for saunas and pergolas, ground screws (helical piles) are a viable alternative. A steel sub-frame is built on top of 4–6 screws and the sauna sits on the frame. Advantages include no concrete pour, no curing time, and reversibility. Costs are higher than a small flat pad but often lower than a complex one. Not suitable for swim spas or large hot tubs without engineering input.
How big should the base be relative to the sauna or hot tub?
For saunas: match the unit's footprint exactly, or extend by 50–100mm. Larger surrounds collect splash-back water and stain the lower walls. For hot tubs and swim spas: match the footprint and add 600mm of paved access on at least one side for service.
Does the base need a damp-proof membrane?
For concrete pads under saunas in damp ground, a DPM under the slab is recommended. For hot tubs and swim spas, it's standard practice. For pergola pad footings, usually not necessary. Your builder will spec this based on ground conditions.
What happens if my base isn't ready on delivery day?
The delivery may still go ahead if there's somewhere temporary to set the unit, but moving it later requires re-craning, which is expensive. The far better outcome is to confirm base readiness with us a week before delivery. If your concrete pour is delayed, contact us early to re-book — most re-books in advance are no-charge.
Do I need a structural engineer for the base?
For standard residential saunas, hot tubs, and pergolas on normal ground, no — a good builder following the spec is sufficient. Engineering input is worth having for: large swim spas, raised installations (deck or balcony), sloping or unstable ground, listed buildings, or any site you're unsure about. The cost of a one-page engineering review is small compared to the cost of getting the base wrong.
Need help spec'ing your base?
Send us your product choice and we'll send you the exact base specification from the installation manual — plus connect you with installers we trust in your area.