How to Build a DIY In-Ground Hot Tub: Digging, Base & Blockwork (2026 Guide)

Updated September 2026. I first wrote this post about digging out my own hot tub. I’ve since helped more than 1,500 people build theirs, so I’ve rewritten it with what I’d tell you today, including a few things I’d now do differently.

Can you really build your own hot tub in the back garden? Absolutely. I did, and it’s the reason this website exists. But the part nobody warns you about is the bit before the fun starts: the dig, the base and the walls. Get these right and everything after (plumbing, tiling, the first soak) is so much easier. Get them wrong and you’re chasing leaks and wonky seats for years.

This guide walks through the groundwork stage of a DIY in-ground hot tub, from the first spade in the ground to a level, watertight shell, using my own build as the example.

Before you dig: plan first

When I built mine, I’d spent what felt like forever planning, and I was desperate to get going. Looking back, the planning was what made the build work, and the bits I rushed are the bits I’d change.

Before a spade goes in the ground, you want to know:

• The internal size and water depth. Most hot tubs sit at around 900mm–1m (36–40in) of water, with seats at a comfortable height for shoulders to be underwater.
• Your wall construction. Block, ICF and poured concrete all have different thicknesses, which changes the size of the hole. There’s more on this below.
• Where the plumbing goes. This means the main drain, skimmer, jets and returns, and the route back to the control room. Make the control room bigger than you think you need. I didn’t, and I’ve had to rebuild mine since.
• Where the electrics come from. A dedicated supply is needed, so plan the cable route before you pour concrete across it.

This is exactly what my DIY hot tub plans are for. You get CAD drawings with every dimension, the plumbing layout, a full parts list and the video course, so you’re not working it out as you go like I was.

Check for buried services

This step isn’t optional. Gas, water, electric and fibre can run through gardens in places you’d never expect.

• UK: Run a free enquiry on LinesearchbeforeUdig and check any plans you have for the house.
• USA: Call 811 a few days before you dig. It’s free, and the utilities will mark their lines.

Permissions

In England, the Planning Portal says you typically won’t need planning permission for a garden hot tub. The exceptions are listed buildings and conservation areas, where you should check with your council. The electrics still have to meet Building Regulations. In the USA it’s a different story, as many cities and counties want a permit for an in-ground spa. I’ve covered this in more detail in Do I need a permit for my hot tub?

How deep do you need to dig?

This is where a lot of first builds go wrong. Your hole isn’t the depth of the water. It’s the water plus everything underneath and around it. Here’s a typical build-up for the floor:

Layer (from the top down)	Typical thickness
Water depth (to the waterline)	900–1,000mm (36–40in)
Freeboard (waterline to the top of the coping)	100–150mm (4–6in)
Tiles, adhesive, membrane and render	~40mm (1.5in)
Reinforced concrete slab	~150mm (6in)
Rigid insulation (optional, recommended)	50mm (2in)
Sand blinding and DPM	~25mm (1in)
Compacted Type 1 sub-base	100–150mm (4–6in)
Total dig depth (flush with the ground)	roughly 1.35–1.55m (53–61in)

Your plans or engineer set the final numbers. The point is that if you dig to the water depth, your seats end up too shallow and you’ll be sitting with your shoulders out of the water. When I wrote the first version of this post, I said to allow for the hardcore and concrete layers so you don’t end up with seats that are too narrow or a tub that isn’t deep enough. That’s still the number one lesson.

How much soil will come out?

More than you think. Take an 8ft x 8ft (2.44m) internal tub with 200mm block walls, dug to about 1.4m with a little working room. That’s roughly 12m³ of soil in the ground, and it bulks up by around a third once it’s dug. So plan for 15–16m³, or two to three 8-yard skips. Sort out where it’s going before day one.

Digging it out: by hand, breaker or digger?

Because of the access to my back garden, there was no way to get a digger round, so mine was dug by hand. It was back-breaking stuff. It took me about two weeks of evenings and full weekends, and there was a lot of rubble and clay to move.

Once I hit the clay and rubble, I bought a mid-range electric breaker for around £200. It was the best money I spent on the whole build. I’d recommend one to anyone digging by hand, although if you know it’ll only be a few days, hiring one is fine too.

What I’d do today: check whether a micro digger will fit. Some are narrow enough to go through a side gate or down an alleyway. Checkatrade’s 2026 cost guide puts micro digger hire at around £125 a day on average (plus VAT, delivery and fuel). A day or two of that against two weeks of my back is an easy choice. If a digger won’t fit, a breaker plus a hired muck-away is the next best thing.

A few digging tips:

• Set out the hole with pegs and string lines, and check the diagonals match so it’s square from the start.
• Dig a little oversize and keep the sides as vertical as your soil allows. Clay stands up well, but sand and loose fill don’t, so don’t work in a deep hole with sides that could collapse.
• Keep the base flat and level. Every millimetre you’re out here, you’ll be fighting in the blockwork later.
• If you hit water, stop and think. A high water table is exactly why the hydrostatic relief valve (below) matters.

The base: sub-base, membrane and a reinforced slab

I was a competent DIYer and had done electrics before, and I’ve always said the plumbing is kitchen plumbing on steroids. What worried me was the structure. Could I build a hole that wouldn’t leak and wouldn’t crack under the weight? My tub holds around 2,800 litres, which is about 2.8 tonnes of water before anyone gets in. So the base has to be right.

1. Type 1 sub-base

Lay 100–150mm (4–6in) of Type 1 (MOT) sub-base and compact it properly. I only put in about 60–70mm and tamped it with a heavy bit of timber and a sledgehammer. It worked, but hiring a plate compactor (“wacker plate”) for a day is cheap and far better. The sub-base spreads the load evenly so the slab doesn’t crack, and cracks mean leaks.

2. Sand blinding and a DPM

On my build I laid a thick PVC pond liner over the gravel. It did the job, but today I’d do what a builder would: a thin layer of sand blinding to protect it, then a proper 1200-gauge polythene damp-proof membrane (DPM), lapped and taped at the joints and turned up the sides.

3. Insulation (the bit I missed)

An in-ground tub loses heat into cold ground all year round. A layer of compression-rated rigid insulation board under the slab and behind the walls makes a real difference to running costs. It’s one of the reasons I’d now build with ICF (more on that below). If you’re heating with an air source heat pump, every bit of insulation helps it keep up. There’s more in What is the best method to heat my DIY hot tub?

4. Rebar, drains and sleeves

I used steel rebar tied together with wire, and I’d still do that. A builder might call it overkill for a hot tub, but I was worried about strength and leaks, and it’s cheap insurance. Typical practice is a grid at around 300mm (12in) centres, sat on spacers so it ends up in the slab and not on the bottom. Leave starter bars sticking up where the walls go so the walls tie into the floor. There’s more detail in my rebar guide.

Before you pour, everything that goes through the floor needs to be in place:

• Main drain(s) and their pipework. See Installing a main drain.
• A hydrostatic relief valve. If groundwater builds up under an empty tub, it can lift or crack the shell. The valve lets that water in rather than letting the pressure build. It matters even more because most paint-on membranes are designed to hold water in from the inside, not keep it out from behind. There’s a full explanation in Why do I need a hydrostatic relief valve?
• Sleeves or ducts for any pipes or cables crossing under the slab.

5. Pour the slab, and use proper concrete

Here’s a confession. On my build I mixed wheelbarrow after wheelbarrow at roughly 4 parts sand to 1 part cement, with plenty of waterproofing additive, working bit by bit from the bottom up. It’s held, but 4:1 sand and cement is really a mortar or screed mix, not structural concrete.

If I did it again, I’d order ready-mix concrete (C25 or C30) and pour the whole slab in one go. That avoids cold joints, which are weak spots where leaks start. If you have to mix on site, use a proper concrete mix with aggregate. A typical C25 mix is about 1 part cement to 1.5 parts sand to 3 parts aggregate. And hire the mixer. I kept putting it off because I didn’t want to pay for it while it sat idle, and I paid for that in sore arms instead.

Tamp and level the slab carefully, keep it damp while it cures, and don’t rush on to the walls. A level slab makes the blockwork far easier.

Walls: block, ICF or poured concrete?

Mine is built from block. It’s perfectly usable, widely available and not difficult. But if I did it again, I’d use ICF (insulated concrete form) without hesitation. I describe it as Lego for adults. It’s lightweight, easy to cut, simple to route plumbing through, and once it’s filled with concrete you get a strong, insulated wall in one go. I’ve written more about this in What I’d change if I built my DIY hot tub again.

• Concrete block (CMU): cheap and familiar. Use hollow blocks with vertical rebar tied into the slab’s starter bars, and core-fill them with concrete.
• ICF: the best choice for most DIY builds. See the best ICF blocks for a hot tub and TubBlox kits.
• Poured concrete or shotcrete: strong, but needs formwork or specialist kit.

There’s a full comparison in Hot tub wall construction: what are the options?

Blockwork tips from someone who’d never laid a block

I hadn’t laid blocks before and I was slow. Very slow, compared with the pros who built my extension. That didn’t matter, because it isn’t a race. What did matter:

• Get the first course square. Mine ended up about 2cm out because I didn’t put guide lines in properly at the start. Measure both diagonals. If they match, you’re square.
• Start from a level base. The deepest part of my tub wasn’t 100% level when I started the first course, and I spent ages compensating. By the second course I had guide lines in and a level base, and it went much better. My top courses finished perfectly square.
• Check level on every block and along every course, not just at the corners.
• Get the mortar consistency right. I mixed mine slightly wetter than normal because I was slow and needed more working time. Too wet, though, and heavy blocks won’t sit properly.
• Put the plumbing in before you core-fill. Pipes through the wall are far easier to set before the concrete goes in, and it’s easier to tie the rebar around them.

Waterproofing: render, membrane, then a flood test

Back then I added plenty of waterproofing liquid to every mix. That helps, but on its own it isn’t a waterproofing system. The modern approach is:

1. Waterproof render on block walls (a scratch coat and a float coat) to give a flat, sound surface.
2. A pool-rated waterproofing membrane, applied in two coats as the manufacturer specifies. In the UK and Europe, Ardex S 7 Plus is made for swimming pools. In the USA, Laticrete Hydro Ban is approved for in-ground pools and water features. Follow the data sheet for primers and cure times.
3. A flood test before you tile. Fill the shell, mark the level and leave it. It’s far easier to fix a leak now than behind finished tiles.
4. Tile on a flexible, pool-grade adhesive. Having seen so many customer builds, I’d now go for mosaic pool tiles over big tiles every time.

Here’s the reassuring part. Out of all the tubs and plunge pools I’ve helped with, very few leak from the structure. Nine times out of ten, a leak is in the plumbing. There’s more in my waterproofing, grout and tile adhesive guide.

Don’t forget the electrics

Plan the supply now, even though it’s connected at the end.

• UK: A new hot tub circuit is notifiable work under Part P. It needs a dedicated circuit with at least a 30mA Type A RCD (an RCBO is the neatest option), usually armoured (SWA) cable for the outdoor run, and a local isolator within sight of the tub but out of reach from it, typically at least 2m away. Use a registered electrician who can certify it.
• USA: The NEC (Article 680) requires GFCI protection and a disconnect within sight of the equipment, generally at least 5ft from the inside wall of the spa. Local adoption varies, so check with your inspector.

There’s more in A guide to hot tub electrics.

What I’d do differently (the quick list)

• Hire a micro digger if it’ll fit, or a breaker from day one if it won’t.
• Use ready-mix C25/C30 for the slab, not sand and cement by the barrow.
• Use a proper DPM, insulation under the slab, and ICF walls instead of block.
• Spend longer on setting out: string lines, diagonals and a level base.
• Build a bigger control room and plan for a dual-pump system from the start.
• Start with a full set of plans rather than working it out as I went.

For more pitfalls, see Avoid these 7 DIY hot tub mistakes. If you want to know how long all this really takes, read How long does a DIY hot tub build take?

Want it drawn up for you?

If one of the standard sizes fits your garden, my hot tub plans give you everything above in CAD, with dimensions, plumbing and a parts list. There are also infinity hot tub CAD plans and swim spa plans.

If your space is an odd shape or you want something truly custom, try the free Tub & Pool Designer Pro software. I can then turn your design into architectural CAD and a 3D plumbing design you can hand to a builder or use for permits.

FAQs

How deep should I dig for a DIY in-ground hot tub?

Dig for the water depth plus everything underneath it. For a typical tub with about 1m of water, that usually means digging roughly 1.35–1.55m to allow for the sub-base, insulation, slab, waterproofing, tiles and freeboard. Your plans set the exact figure.

How thick should the concrete floor be?

For an in-ground tub, a reinforced slab of around 150mm (6in) is typical. A surface pad for a shop-bought hot tub can be thinner (see how to build a concrete slab for a hot tub), but an in-ground tub is holding water, so give it more.

Do I need rebar?

I’d always use it. A steel grid on spacers in the slab, tied into vertical bars in the walls, turns the floor and walls into one structure and resists cracking.

Can I dig a hot tub by hand?

Yes. I did. Budget a couple of weeks of evenings and weekends, buy or hire a breaker for clay or rubble, and plan where the spoil is going.

Do I need planning permission in the UK?

Usually not for a garden hot tub, unless you’re in a listed building or a conservation area. The electrics must still meet Building Regulations (Part P).

Got a question about your own build? Drop it in the comments below. I read them all.

Cheers,
Andi

You can also find me on my socials;

Can I Help You?

If I can help you in any way I would love to hear from you. You can get in touch using the form below.

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