Sunken hot tubs and how to sink one properly

A sunken hot tub sits partly or fully below the level of the ground or deck around it, so you step down into it instead of climbing over a tall side. For most portable cabinet spas, a full inground install causes problems, but a semi-sunken or deck-inset install gives you the same low look while keeping the equipment reachable.

This guide explains the different install types, how much space to leave around the cabinet, and how to deal with drainage, ventilation and retaining walls. The aim is a pit that stays dry and a tub that a technician can still repair in ten years.

What is a sunken hot tub, and can you sink any hot tub?

A sunken hot tub is any tub set lower than the ground or deck around it. The term covers three quite different jobs: a fully inground spa, a semi-sunken install where the tub sits in a shallow lined pit, and a deck-inset install where a raised deck is built around a tub standing on a ground-level base.

True inground spas are usually built from concrete, tiled blockwork or a shell designed for burial, with the pumps and heater housed separately in a plant area. They are closer to building a small pool than installing a hot tub, and they are planned in at the design stage of a garden.

Purpose-built inground spas come in two main forms. One is a custom shell in concrete or sprayed concrete, which can be built to almost any shape. The other is a prefabricated acrylic shell lowered into an excavated pit. The custom concrete route usually costs considerably more.

A true inground spa is often built beside a swimming pool as a spillover spa. The spa water flows over one edge and cascades into the pool, and the two usually share the same plant equipment.

For a true inground spa, hire a specialist pool and spa contractor rather than a general landscaper. The job involves the shell, the plumbing, a separate plant area and the electrics, and each part has to work with the others.

Most hot tubs sold in the UK are portable cabinet spas. The shell, pipework, pumps, heater and control box all sit inside a wooden or composite cabinet, and they expect air around them and a technician able to remove the side panels. Burying that cabinet in soil traps moisture, hides leaks and turns every repair into a dig.

That is why semi-sunken and deck-inset installs are the realistic choice for most gardens. You get the low, built-in look, and the cabinet stays dry, ventilated and reachable. If you are still deciding on the broader question, our guide to above ground and inground hot tubs compared covers the wider trade-offs.

Which type of sunken install suits your garden?

Each install type trades effort against appearance. The table below compares the main options for a standard portable cabinet spa.

Deck-inset is the lowest-risk way to get a sunken look, because the tub never sits below the surrounding soil. Rain drains away under the deck and the base stays at ground level. Our hot tub decking ideas for UK gardens shows several ways to frame a tub like this.

Semi-sunken works well on sloping gardens, where cutting into the slope gives you a low edge on one side without building a high deck. It involves more groundwork, so drainage and retaining walls need proper planning from the start.

Allow for build time in your plans. A purpose-built inground spa usually takes several weeks, roughly two to six, while a portable tub on a ready base can go in on the day it is delivered. A semi-sunken or deck-inset install sits in between, because the groundwork, walls and slab must be finished before the tub arrives.

Think about how long you will stay. Sinking a portable tub or building a deck around it effectively makes it permanent. Lifting it out to move house or to put it somewhere else in the garden can mean removing deck sections, hiring a crane or lifting gear and making good the pit, which is a much bigger job than moving a tub that sits on a surface base.

How much access do you need around the cabinet?

Almost every repair on a portable spa happens through the cabinet panels. Pumps, the heater, the control box, unions and valves all sit behind them, and leaks often show first as damp on the inside of a panel. If a technician cannot remove the panels, a small fix can mean lifting a deck or digging out soil.

Check the manual for your model first. Many manufacturers state which panels must stay removable and how much clear space they want around the cabinet, and some make this a condition of the warranty. Requirements differ between brands, so follow the figure for your tub rather than a general rule.

Where the manual gives no figure, plan a gap wide enough for an adult to kneel in and work with both hands. Give the most space to the equipment side, which is usually shown in the manual or marked on the cabinet.

• Keep the equipment side fully open or behind a lift-out deck section.
• Use deck panels that lift out by hand, not ones screwed down or covered by planters and furniture.
• Leave room to pull each panel straight out, not just to unclip it.
• Make sure the drain valve and filter compartment are reachable without moving anything.
• Keep a route to lift the tub out in future, for example through a gate or over a low wall.

In a semi-sunken pit, the simplest approach is a service trench: a gap between the cabinet and the pit wall, covered by removable grating or deck boards. It uses more space, but it keeps the tub serviceable for its whole life.

For a sunken install, choose a brand or dealer with good local service support and a reliable supply of parts. A tub that is harder to reach depends even more on repairs being quick and simple, so ask who will service it and how long common parts take to arrive.

When choosing a tub for a sunken install, look closely at the pumps and heater. These parts largely decide how long the tub lasts and how often someone has to get into the cabinet to replace them. Ask the dealer who makes them, what warranty covers them and how easy they are to swap, since every replacement means opening up the access.

How do you keep a sunken hot tub pit dry?

Drainage is where most sunken installs go wrong. A pit is a hole that collects water, and in clay soils it can fill after heavy rain and stay full for days. Standing water around a cabinet rots timber, corrodes fittings and puts the electrics at risk.

Before you commit, dig a test hole to the depth you plan, fill it with water and watch how quickly it drains. Look again after a spell of heavy rain. If water seeps in from the sides or bottom, the groundwater level is high and a deep pit will need active drainage.

• A layer of compacted hardcore and gravel under and around the base, so water moves away from the slab.
• A perforated land drain around the pit, laid to a fall and run to a soakaway or a suitable outlet.
• A sump at the lowest point with a float-switch pump, where the ground cannot drain naturally.
• A slab laid with a slight fall towards the drain, so water never sits under the cabinet.
• Paving or decking around the pit laid to fall away from it, so rain runs outwards, not inwards.

Plan where the tub water goes when you empty it, too. Draining a full tub into a pit with poor drainage simply floods it. Treated water usually belongs in the foul drain, not a surface water drain, ditch or stream, so check with your water company if you are unsure.

Keep the tub lip slightly higher than the surrounding surface, even on a flush-looking install. A small upstand stops rainwater, soil and garden runoff washing into the water, and it gives people a clear edge so they are less likely to step in by mistake.

A sump pump needs its own electrical supply and regular checks. Test it each autumn by pouring water into the sump and making sure the float switch starts the pump.

Why does a sunken cabinet need ventilation?

Pumps, the heater and the control box all give off heat, and the warm water in the shell adds moisture. On a surface install that air escapes easily. In a pit or under a deck it can collect, cool and condense on timber, metal and electrical parts.

Some tubs are designed to reuse motor heat inside a closed cabinet, so do not cut extra vents in the cabinet without checking the manual. The aim is to stop damp air building up around the outside of the cabinet, not to change how the tub was designed. The manual will say which grilles must stay clear.

• Condensation on the underside of deck boards or access covers.
• Mould, green staining or soft timber on the cabinet panels.
• Rust on screws, brackets or the equipment frame.
• A damp or musty smell when you lift a service panel.
• The RCD tripping after wet weather.

Gaps between deck boards, a ventilated skirting around the pit and a service trench open to the air all help. Never pack insulation, soil or membrane tightly against the cabinet to fill a gap, as this holds moisture against the panels.

When do you need a retaining wall?

Whenever the ground around the tub is higher than the base, the soil pushes sideways. Wet soil pushes harder than dry soil, and clay swells and shrinks with the seasons. Without a wall, the pit edges slump, soil spills onto the base and the cabinet ends up against wet earth.

The wall must hold back the soil, not the tub. A hot tub cabinet is not built to resist ground pressure, so keep the service gap between cabinet and wall and never backfill against the panels.

• Concrete block walls on a concrete footing, the usual choice for a permanent pit.
• Timber sleepers, quicker to build but shorter-lived in constantly damp ground.
• Gabion baskets filled with stone, which drain freely and suit sloping gardens.

Every retaining wall needs drainage behind it. Gravel backfill with a perforated pipe at the base, or weep holes through the wall, lets water escape instead of building up. Trapped water adds pressure, and it is a common reason garden walls lean or crack.

For a deep pit, or one close to a house, a boundary or a slope, or in clay or made-up ground, ask a structural engineer to design the wall. Local authority building control can tell you whether approval is needed. If you will dig near a neighbour's wall or foundations in England or Wales, read the GOV.UK guidance on party walls and building work first.

What base does a sunken hot tub need?

A filled hot tub is heavy. 1,500 litres (about 400 US gallons) of water weighs around 1,500 kg (3,300 lb) before you add the shell, the cabinet and the people in it. That load needs a level, solid base that will not settle unevenly.

With four adults in it, a filled tub can weigh more than about 1,800 kg (4,000 lb). Size the base for the water, the tub and the people together, not just the water.

In a pit, a reinforced concrete slab on compacted hardcore is the usual choice, because it stays level when the ground around it is damp. Some manufacturers state a minimum slab thickness or ask for full support under the whole footprint, so check the manual. Our guide to hot tub base options compares slabs, pads and paving in more detail.

Where the manual gives no figure, a reinforced concrete slab at least 100 mm (4 in) thick, laid level and with drainage, is the usual minimum base for a permanent or semi-sunken tub.

A typical garden deck is built for far less load per square metre than a filled hot tub puts on it, often around half or less. Never stand a tub on a raised deck unless a structural engineer has confirmed the deck can take it. The safer approach is to set the tub on its own slab and build the deck around it.

As a rough guide, a typical garden deck is designed for about 1.9 to 2.9 kN/m² (about 40 to 60 lb per sq ft). A filled hot tub with people in it can put more than about 4.8 kN/m² (100 lb per sq ft) on the area beneath it. Actual figures depend on the deck design and the tub, so an engineer should work from your tub's filled weight and footprint.

A level slab in a pit with nowhere for water to go becomes a tray. Always pair the slab with the drainage layer and outlet described above, so the base sheds water instead of holding it.

How do you sink a hot tub properly, step by step?

The order matters more than any single step. Electrics, drainage and walls are far easier to get right before the tub arrives than after. Our full guide on how to install a hot tub covers delivery, positioning and the first fill.

1. Read the manual for your model and note its base, clearance, ventilation and access requirements.
2. Check planning rules, boundaries and buried services such as drains, gas, water pipes and cables before you mark out.
3. Dig a test hole and check how the ground drains, including after heavy rain.
4. Mark out the pit to the tub footprint plus a service gap on every side that needs access.
5. Excavate to depth, allowing for hardcore, drainage gravel and the slab.
6. Lay the drainage, such as a land drain, sump or soakaway, and a duct for the electrical cable.
7. Build the retaining walls on proper footings, with drainage behind them.
8. Lay and compact the hardcore, then pour a level, reinforced slab.
9. Have a registered electrician run the supply and fit the isolator and RCD protection.
10. Lower the tub into place, check it is level, and confirm every service panel can be removed.
11. Build the deck or paving surround with lift-out access sections and a slight fall away from the tub.
12. Fill and start the tub, then check inside the cabinet for leaks before closing the access panels.

Be patient after the first fill. A typical tub heats by only about 1 to 1.5°C (2 to 3°F) per hour, so reaching 40°C from a cold fill can take a day or more. Keep the cover on while it heats, and plan the first soak for the following day.

Allow extra time if you fill the tub in winter. Heating from a cold fill takes noticeably longer in cold weather than in summer, because the tap water starts colder and the tub loses more heat to the air while it warms. Start heating earlier and leave more time before the first soak.

Pit sides can collapse with little warning, especially in wet or sandy ground. Treat any excavation as unstable until the sides are supported, keep spoil heaps and machinery back from the edge, and never work alone in a pit. The HSE construction pages explain how trades manage excavation risks.

For a sense of what each stage adds to the bill, see our breakdown of hot tub installation costs in the UK. Groundwork, walls and drainage usually make a sunken install noticeably more expensive than placing the same tub on a surface base.

What about electrics, planning and safety?

Electrics

In the UK, a hot tub supply must follow BS 7671, the national wiring regulations, with RCD protection and a suitable isolator. In England and Wales, a new outdoor circuit is notifiable under Part P of the Building Regulations, so use a registered electrician who can self-certify the work. Other countries have their own wiring rules, so follow local regulations outside the UK.

A sunken install adds details to agree early: the cable route through ducting in the groundwork, an isolator placed out of reach of anyone in the water but easy to reach in an emergency, and a separate supply for any sump pump. Electrical Safety First has clear guidance for householders on outdoor electrics.

Planning

In England, a hot tub on its own usually does not need planning permission. A deck or platform more than 30 cm (about 12 in) above the ground usually does, and listed buildings, conservation areas and flats have tighter rules. Sinking the tub can help keep a surrounding deck under that height. Rules differ in Scotland, Wales and Northern Ireland, so check with your local planning authority.

Safety around a low edge

A low lip is easier to fall into, especially for young children and pets. Fit a lockable cover and keep it locked whenever the tub is not in use. Keep the water no hotter than 40°C (104°F), and supervise children at all times.

Because a low-edged tub is easy for children to reach, consider a fence or barrier around the tub area as well as the lockable cover. Fit it with a self-closing, self-latching gate, so it shuts and locks itself even if someone forgets.

Make the fence or barrier at least about 1.2 m (4 ft) high, so a small child cannot climb over it. Avoid horizontal rails or gaps that work as footholds, and keep garden furniture and planters away from the outside of the fence.

The edge of a sunken tub is also a trip hazard. Use a non-slip surround, light the edge and any steps for evening use, and choose an edge strip in a contrasting colour so people can see where the deck ends and the water begins.

Anyone with a heart or circulation condition, chronic pain or another health condition should ask their GP before using a hot tub, especially if they plan to use it for therapeutic reasons. Stop and get out if you feel dizzy, faint or unwell. The NHS website has information on many conditions to read before that conversation.

Sinking a hot tub: the short version

For most portable cabinet spas, a semi-sunken or deck-inset install gives you the low look of an inground tub without burying the equipment. Plan access to every service panel, a dry pit with a clear drainage route, air movement around the cabinet and proper retaining walls before you dig.

Follow the manual for clearances and base requirements, use a registered electrician for the supply, and bring in a structural engineer for deep pits or walls near buildings. Done in that order, a sunken hot tub is as easy to live with as one on the patio.

Frequently asked questions

Can you sink an inflatable hot tub?

It is not a good idea. Inflatable tubs need a flat, clean, dry surface, and setting them into the ground traps moisture, grit and sharp stones against the liner. Most manufacturers say to stand them on a level surface, so check your manual. Inflatables also typically last only about 3 to 5 years even with good care, so a permanent pit makes little sense. For a lower look, build a simple deck around the tub instead, leaving open space around the pump unit.

How deep should you sink a hot tub?

There is no single right depth. Many owners set the lip at roughly seat height, so you can sit on the edge and swing your legs over, which is easier than climbing in and safer than a fully flush edge. Whatever depth you choose, keep the lip slightly above the surrounding surface so rainwater and soil stay out of the water.

How much does a sunken hot tub cost to install?

It depends on the ground, the depth, the walls and the electrics, so prices vary widely between gardens. A semi-sunken install usually costs noticeably more than placing the same tub on a slab, because of excavation, spoil removal, drainage and retaining walls. Budget separately for finishing the landscaping, such as paving, planting and making good the lawn. A dedicated circuit can also mean upgrading the consumer unit or supply, so ask for itemised quotes with any electrical upgrade as its own line.

How long do inground hot tubs last?

Most inground spas last roughly 5 to 15 years, and well-built acrylic or properly installed concrete spas can reach 15 to 20 years or more, although pumps and heaters will need replacing along the way. A portable tub in a pit should last as long as one on a patio if the pit stays dry and ventilated. Consistent water chemistry, regular filter cleaning and prompt repairs extend any tub's life most.

Does sinking a hot tub affect the warranty?

It can. Some manufacturers set installation conditions, such as removable panels, a minimum clearance or a particular type of base, and a tub that cannot be serviced may not be covered. Read the warranty terms before you dig, and keep photos of the base, drainage and access arrangements in case you need to show how the tub was installed.

Does a sunken hot tub keep its heat better?

Sometimes slightly, because a pit or deck shelters the cabinet from wind. Damp ground can also draw heat away, so a wet pit may cancel out any gain. The cover does most of the work of keeping heat in, so a well-fitting, insulated cover in good condition matters far more than how low the tub sits. The tub's own insulation also counts: a well-insulated shell and cabinet, such as full-foam, holds heat better in cold weather and runs the heater less, so check it when choosing a tub to sink. An energy-efficient circulation pump also lowers running costs over the years, because it runs for many hours each day, so ask the dealer about pump efficiency too.

Do retaining walls for a hot tub pit need approval?

Small garden retaining walls often do not, but it depends on height, location and what the wall supports. A wall holding up a path, a building, a boundary or a neighbour's ground may need building control approval or an engineer's design. Speak to your local authority building control before building anything deep or close to a property.