In-Ground Hot Tubs: A DIYer’s Guide to Planning and Building One

Last updated: September 2026

An in-ground hot tub is my favourite kind of hot tub. I built one in my own back garden, and it’s the reason this website exists. Since then I’ve helped more than 1,500 people build their own, from small block tubs in suburban gardens to big ICF spas in American backyards.

This is the overview. It covers what an in-ground hot tub is, the decisions you need to make before you dig, the construction methods, the plumbing, heating and electrics, and the mistakes I see most often. Where a topic needs more depth, I link to the detailed guide, so you can use this page as your map.

What is an in-ground hot tub?

An in-ground hot tub is a hot tub built into the ground, or into a raised patio or deck, so the water sits roughly level with the surface around it. Instead of a moulded acrylic shell in a cabinet, it’s a structure you build in place: a concrete slab, walls of block or ICF, a waterproof membrane and a tiled finish, with the pumps, filter and heater in a separate control room nearby.

One thing I’d steer you away from is buying a plastic shell hot tub and dropping it into a hole. They’re designed to sit above ground, with the cabinet supporting the shell and giving access to the plumbing. In the ground, you lose that access, and water and soil pressure work against the shell. If you want an in-ground tub, build one. It’s easier than you think.

In-ground, above ground or semi-sunken?

Option	What it involves	Good for	Watch out for
In-ground	Dug in so the coping is flush with the patio or lawn	The cleanest look, and less on display for the neighbours	The most digging, groundwater, and a trip hazard if it isn’t covered or fenced
Semi-sunken	Partly dug in, with a wall at seating height above ground	A good compromise: less digging, and the wall doubles as a seat	The exposed part of the wall needs insulating and finishing
Above ground (built)	A block, ICF or timber tub built on a slab, with steps up	High water tables, rock, or no way to get a digger in	The walls have nothing behind them, so they need proper reinforcement

If you’re leaning towards a raised build, see Above ground hot tubs: how to build your own. For inspiration on shapes and finishes, have a look at What makes for good in-ground hot tub designs?

Your in-ground hot tub, stage by stage

Here’s how a build breaks down, and where to read more on each stage.

Stage	What happens	Detailed guide
1. Design	Size, shape, seats, finishes, control room, heating	In-ground hot tub designs
2. Groundwork	Services check, dig, sub-base, drains, relief valve, rebar, slab	DIY in-ground hot tub build guide
3. Walls	Block, ICF or poured concrete, with the wall fittings set in	Hot tub wall construction
4. Plumbing	Suction, returns, jets, skimmer, pressure testing	DIY hot tub plumbing
5. Waterproofing and tiling	Render, membrane, flood test, tiles	Waterproofing, grout and tile adhesive
6. Equipment and electrics	Spa pack, pumps, filter, heat pump, qualified electrician	A guide to hot tub electrics

Design decisions to make first

This is the enjoyable bit, where you get to bring the idea to life. But every decision here affects the dig, so make them before a spade goes in the ground.

Location

• Privacy and the view. Pick a spot you’ll actually want to sit in, facing a view rather than a fence if you can.
• Close to the house. You’ll use it far more in winter if it’s a short walk from the back door.
• Shelter and trees. Wind strips heat from the water, and overhanging trees fill the skimmer with leaves.
• Access for the build. Can a micro digger get in? Where will the spoil go? Mine was dug by hand because there was no way to get a digger round, and it was back-breaking.
• Services. Check for buried cables and pipes first. In the UK use the free LinesearchbeforeUdig service, and in the USA call 811 a few business days before you dig.
• Power, water and drainage. Plan the cable route, somewhere to fill it from and somewhere to empty it to before you pour any concrete.

Size and seating

Size depends on your space, how many people will use it and how you like to sit. Bigger tubs are more sociable. Smaller ones are cosier and cheaper to heat. Personally, there’s nothing worse than bumping into other people in the tub, so I like having my own space. Most tubs work well with around 900mm–1m (36–40in) of water, and seats set so your shoulders are under the water when you sit back. Remember that the hole is deeper than the water, which is covered in the groundwork guide.

Shape

Squares and rectangles are by far the easiest shapes to build in block or ICF, and the easiest to tile and cover. Circles, ovals and curves are possible, but they’re harder to build and tile, and the cover will need to be made to measure. The plumbing is the same whatever the shape, so pick something you can build well.

Finishes

I’d use swimming pool tiles every time. They cost more than paint or a liner, but they look fantastic and last. Mosaic tiles follow curves and steps better than big tiles, and they’re less slippery underfoot. Whatever you choose, make sure the tile, adhesive and grout are rated for permanent immersion in pools. There’s more in my hot tub tiles guide.

Construction methods

There are three realistic DIY options, plus one that’s best left to the pros.

Method	Pros	Cons	DIY-friendly?
Concrete block (CMU)	Cheap, available everywhere, and familiar to any builder	Heavy, slow if you’ve never laid blocks, and no insulation of its own	Yes. Use hollow blocks, vertical rebar and core-fill
ICF (insulated concrete form)	Lightweight, quick, built-in insulation, easy to route pipes through	The concrete pour needs care and bracing	Yes. My top pick
Poured concrete	Very strong and seamless, and any shape is possible	Needs well-braced formwork and a well-planned pour	With help
Gunite or shotcrete	Strong and free-form, and the standard for pools	Needs specialist spraying kit and an experienced crew	No. Hire a specialist

Mine is built from block, and it’s fine. But if I built again, I’d use ICF without hesitation. It’s like Lego for adults: the forms dry-stack, one person can build the walls, and once they’re filled with concrete you get a strong, insulated wall in one go. The insulation also helps keep the heat in, which matters on a tub sitting in cold ground all year. See the best ICF blocks for building a hot tub, or if you’re set on block, Cinder block hot tub: a DIYer’s guide.

Structural must-haves

Whatever the walls are made of, an in-ground tub holds a lot of water. Mine holds around 2,800 litres, which is 2.8 tonnes before anyone gets in. The essentials:

• A compacted sub-base, a membrane and a reinforced concrete slab, with rebar tied into the walls.
• A hydrostatic relief valve, so groundwater can’t lift or crack the shell when it’s empty.
• Main drains with anti-entrapment covers.
• Waterproof render and a pool-rated membrane, such as Ardex S 7 Plus, then a flood test before you tile.

The step-by-step for all of this is in the DIY in-ground hot tub build guide.

Plumbing considerations

Put the control room below the waterline

The control room, or equipment room, houses the pumps, filter, heater and spa pack. Standard hot tub pumps are centrifugal pumps, and they aren’t self-priming. They rely on water flowing to them by gravity. So the easiest arrangement is to put the control room at or below the level of the water in the tub, so the pumps are always flooded and primed.

If the equipment has to sit above the waterline, it can be done, but you’ll need a self-priming pump or a priming arrangement. See What are the self-priming hot tub pump options?.

Make the control room bigger than you think, keep it close to the tub, give it a drain, and leave access to every joint. I didn’t leave enough room in mine and have since had to rebuild it. See control room design considerations.

Number of jets

The number of jets is limited by the flow the pump can deliver. As a rule of thumb from our builds, up to about 16 jets can run on one pump if the control room is close to the tub. Longer pipe runs, smaller pipe and higher-flow jets all reduce that number. If you want more jets than that, add a second pump. See how to correctly size a hot tub pump.

Jet placement

Where the jets go changes how the tub feels. Clusters of jets aimed at the back, shoulders or feet give a focused, intense massage. Jets in a line give a gentler, more even feel. Most good designs mix the two, with each seat different, so people can pick the one they like.

Keep the circuits separate

The filtration and heating circuit should run separately from the jet circuit, with a skimmer at the waterline to pull surface debris into the filter. My plumbing guide, DIY hot tub plumbing, explains how it all fits together.

Heating and insulation

An in-ground tub loses heat into the ground all year, so insulation under the slab and behind the walls pays for itself. On the heating side, a spa pack heater works, but an air source heat pump is far cheaper to run. See DIY hot tub insulation and how to choose the right air source heat pump.

Then there’s the cover. A flush in-ground tub suits a walk-on cover, because people will step on it. Design the tub around a cover you can actually buy or make. My guide to hot tub covers has the options.

Electrics

• UK: A new hot tub circuit is notifiable work under Part P. You need a dedicated circuit with at least a 30mA Type A RCD, SWA cable for the outdoor run, and an isolator that’s visible from the tub but at least 2m from it. Use a registered electrician who can certify it.
• USA: NEC Article 680 requires GFCI protection, a disconnect within sight of the equipment (generally at least 5ft from the water) and equipotential bonding. Local adoption varies, so check with your inspector.

Plan the cable route and the bonding before you pour concrete. See Hot tub wiring explained.

Permits and safety

• UK: The Planning Portal says you typically won’t need planning permission for a garden hot tub, but check with your council if you’re in a listed building or conservation area. The electrics still have to meet Building Regulations.
• USA: Many cities and counties want a permit for an in-ground spa. The CPSC’s barrier guidelines include spas and hot tubs, and accept a powered safety cover meeting ASTM F1346 as an alternative in some cases. See Do I need a permit for my hot tub?

Wherever you are, a flush tub is easy to fall into, so a lockable cover is a must if children are ever around.

Common mistakes

• Digging to the water depth rather than the water depth plus the build-up underneath, which leaves the seats too shallow.
• A control room that’s too small, too far away or above the waterline without a plan for priming.
• Too many jets on one pump, which leaves them all feeling weak.
• Skipping the hydrostatic relief valve, the membrane or the flood test.
• Mixing imperial and metric pipe. They look alike but don’t fit properly.
• Leaving the cover as an afterthought, then finding nothing fits an unusual shape.

There are more in Avoid these 7 DIY hot tub mistakes. If you’re at the very start, read Planning a DIY hot tub in 2026: 10 key decisions.

Planning your own build?

My DIY hot tub plans give you everything above worked out for you: CAD drawings with every dimension, the plumbing layout, a full parts list and the video course. There are also infinity hot tub plans if you want an overflow edge.

If your garden is an odd shape or you want something truly custom, draw it in the free Tub & Pool Designer Pro software, and I can turn it into architectural CAD and a 3D plumbing design.

FAQs

Can I put a normal hot tub in the ground?

I wouldn’t. Acrylic shell tubs are designed to sit above ground in their cabinet, with access to the plumbing. Sunk into a hole, you lose that access and the shell isn’t designed for the soil and water pressure around it. Build a proper in-ground tub instead.

What’s the best material for an in-ground hot tub?

For most DIYers, ICF. It’s light, quick to build and insulated. Core-filled concrete block works well too. Gunite and shotcrete are best left to specialists.

How deep should an in-ground hot tub be?

Around 900mm–1m (36–40in) of water suits most people. The hole needs to be deeper, typically 350–400mm (14–16in) more, to allow for the sub-base, slab, waterproofing, tiles and freeboard.

Where should the pumps go?

In a control room close to the tub, at or below the waterline so standard centrifugal pumps stay primed. If that isn’t possible, use a self-priming pump.

How many jets can I have?

As a rule of thumb, up to about 16 per pump with a close control room. More than that, add a pump.

Do I need planning permission for an in-ground hot tub in the UK?

Usually not for a garden hot tub, unless you’re in a listed building or a conservation area. The electrics are still notifiable under Part P.

Planning your own in-ground tub? Ask me anything in the comments below. I read them all.

Cheers,
Andi

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Thanks - Andi

 

Hi, Andi here. I own Buildahottub.com and also write all of the articles and info pages on the site. Some years back now, I built my own hot tub but struggled to find the information I needed. So, once my tub was complete, I started this website to help others in their own pursuit of hot tub and plunge pools DIY building information.

Fast forward to 2025, I've helped over 1400+ DIY customers just like you all over the world build hot tubs and pools. Have a good look around the site, there are lots of resources here. Please do get in touch if I can help you. - Cheers, Andi