How to Build a DIY Cold Plunge Pool: Size, Walls, Chiller and Safety

Last updated: September 2026

If you know me, you’ll know I’m not a cold plunge fan. I’m a hot tub kind of guy and I like my water bubbling. Freezing cold water is not for me! But plenty of you want to build one, and I’ve helped quite a few people do it. Building a cold plunge is basically the same job as building a hot tub. You’re just fitting a chiller instead of a heater.

In this guide I’ll cover what you need to think about when designing and building a DIY cold plunge: where to put it, how big to make it, what to build the walls from, the plumbing and chiller, water treatment, and, most importantly, how to use it safely.

What I mean by a DIY cold plunge

Let me be clear first. The cold plunges I’m talking about aren’t bath tubs or chest freezers filled with ice or cold water. For me, pumps and filtration are what make a hot tub a hot tub, and the same goes for a cold plunge.

The builds I design are proper “spa-like” plunge pools, chilled by a physical chiller. They’re filtered and sanitised, and a pump turns the water over to keep it fresh and healthy, so you can use the same water for months. A single-use tub is, in my mind, a waste of water.

Where should it go?

Think about location before anything else:

• Somewhere warm to escape to. You’re not going to sit in a cold plunge for hours, and you’ll want a shower, a sauna, a hot tub or at least a warm room close by. Make sure there’s space for that.
• Water and drainage. You’ll need to fill it, top it up as bathers splash water out, and drain it occasionally. A hose connection nearby is the minimum. Plan where the drain water goes.
• Power. The pump and chiller need a proper supply. More on that below.
• Shade for the chiller and the tub. A plunge in full summer sun is fighting the chiller all day. A chiller also needs airflow around it to get rid of the heat it pulls out of the water.
• Privacy and access. Level, non-slip paths, and somewhere to hang a towel and a robe.

Plenty of people put a cold plunge next to a hot tub for hot and cold contrast. If that’s your plan, have a look at Kyle’s cold plunge and hot tub combo and Patrick’s basement hot tub and cold plunge.

How big should a DIY cold plunge be?

That comes down to the space you have and how many people will use it at once. Unlike a hot tub, a cold plunge usually isn’t a social space. You’re not sitting in there with friends having a drink. Most are designed for one person, or two at most.

• Depth: typically 0.9–1.5m (3–5ft), enough to get your shoulders under comfortably.
• Layout: most people stand or lie down in a cold plunge rather than sit, so the norm is no seats. I have seen a few with a bench seat, and a single step or ledge makes getting in and out much safer.
• Volume: smaller really is better here. For example, an internal size of 1.8m x 0.9m (6ft x 3ft) with 1.1m (43in) of water holds about 1,780 litres (470 US gallons). That’s less water to chill, less to treat and a smaller chiller to buy.

If you’d rather size it visually, the free Tub & Pool Designer Pro software lets you draw it up and see the volume.

What should you build the walls from?

A cold plunge full of water puts a lot of pressure on its walls, so the structure matters just as much as it does on a hot tub. You also have to play to your strengths. This is a DIY project and there’s more than one way to do it. I like timber and I like block, but I wouldn’t take on a poured concrete cold plunge myself, as that’s not really where my skills lie.

Poured concrete (formwork)

You build a mould from timber and fill it with concrete to cast the walls (and seats, if you want them). The result is a perfectly cast shape, and because the plumbing is set in the formwork before the pour, you get a good seal around every fitting. The catch is the bracing. Wet concrete creates a huge amount of pressure, so the formwork needs a lot of bracing, or you’ll end up with bowed walls.

Concrete block with a cavity (dual skin)

Concrete block is one of the easiest and most reliable ways to build. With this method you build two skins of block with a cavity between them. The cavity takes your plumbing, insulation and a rebar cage, and it’s then filled with concrete, which must be properly vibrated for a solid fill. The downside is a wide wall. A 4in block, 6in cavity and another 4in block makes at least 14in (about 350mm), before you add the coping. It’s strong, well insulated and looks great once it’s capped.

Hollow concrete block (CMU)

CMU blocks are the hollow concrete blocks you see on most building sites. They’re cost-effective and strong. Rebar is set into the concrete base and runs up into the walls, tying the whole structure together. Put rebar in every other hollow core (or as your plans or engineer specify), then fill the cores with concrete. Horizontal reinforcement is optional but adds strength. There’s more in my rebar guide.

ICF (insulated concrete form)

ICF blocks are hollow foam blocks that you stack dry, reinforce with rebar and fill with concrete. The foam stays in place, so the wall is insulated on both faces. They slot together like Lego, which makes them quick and easy for a DIYer, and insulation is especially useful on a cold plunge. The main downside is the cost, which is usually higher than block. See what to look for in ICF blocks and my ICF wall guide.

An existing hole in the ground

I’ve done a number of projects where there was already a hole the customer wanted to convert, often where an old sunken plastic tub has come out. That’s fine. You build a single skin of 4in block inside the hole with a 6in cavity behind it, put the plumbing and rebar in the cavity, then fill it with concrete.

Timber

Cedar or other durable timber makes a lovely above-ground cold plunge and is well within most DIYers’ skills. It works the same way as a DIY wooden hot tub.

Which method should you choose?

Method	Good for	Watch out for
Poured concrete	Custom shapes, well-sealed plumbing	Formwork and bracing skills, timber cost
Dual-skin block	Strength, insulation in the cavity	Wide walls, lots of blocklaying
CMU block	Low cost, widely available	Add insulation separately
ICF	Speed, built-in insulation, solo builders	Higher material cost, bracing the pour
Timber	Above-ground, woodworkers	Needs a liner or careful joinery, ongoing upkeep

All of these give a fantastic result. Pick the one that suits your skills, your budget and what you can get locally. There’s a full comparison in Hot tub wall construction: what are the options? For in-ground builds, the base, waterproofing and flood test are the same as for a hot tub, so follow my in-ground hot tub build guide, including a hydrostatic relief valve if you’re below ground.

Insulation: yes, even for a cold plunge

It sounds obvious, but people skip it. Insulation works both ways. It keeps the cold in and the heat out, especially in summer. Insulate the walls and floor, and fit a good insulated cover. It cuts how hard (and how long) the chiller has to run, so the plunge doesn’t end up costing you the earth. My DIY hot tub insulation guide applies here too.

Plumbing and the chiller

The plumbing is a small hot tub circuit without the jets. You’ll need:

• A skimmer to take surface debris, plus two main drains in the floor. Twin drains at least 1m (3ft) apart, with anti-entrapment covers, mean nobody can be held against a single suction point. See Installing a main drain.
• A pump to turn the water over.
• A filter. A cartridge filter is simplest on a small volume. Sand works on larger plunges. See cartridge or sand filter?
• A chiller, plumbed after the filter so it only sees clean water.
• Two returns to bring the chilled water back and keep it circulating.
• Optional UV or ozone, and a way of dosing sanitiser.

Choosing a chiller

A plunge chiller works like a fridge (or a heat pump in reverse). It pulls heat out of the water and dumps it into the air around it. What size you need depends on the water volume, how cold you want it, how warm your climate is, and how well the plunge is insulated and covered. A bigger or hotter setup needs more cooling capacity.

• For small plunges, you can buy all-in-one units with the pump, filter and chiller in one box. They’re simple, but they’re only meant for small volumes, so check the maximum capacity on the spec sheet against your plunge.
• For anything bigger, or a built-in plunge, you’ll want a separate chiller plus a proper plumbing kit: pump, filter, drains, returns and valves, sized for the volume.
• Some pool heat pumps are reversible and can cool as well as heat. If you’re considering one, check the cooling output and minimum water temperature on the spec sheet. My guide to choosing an air source heat pump explains how they’re rated.

Put the chiller in shade with plenty of airflow. If it’s boxed in, it recirculates its own warm air and struggles.

Electrics

Treat the electrics exactly as you would for a hot tub. You need a dedicated circuit with RCD protection (GFCI in the USA), installed and certified by a qualified electrician. In the UK a new circuit like this is notifiable under Part P. My hot tub wiring guide covers the details for both the UK and the USA.

Keeping the water clean

Cold water slows bacteria down, but it doesn’t stop them, and every plunge brings in sweat, skin and whatever’s on your feet. So:

• Filter every day, and clean the filter regularly.
• Use a sanitiser (chlorine or bromine) and test it. The same ranges as a hot tub are a safe target. See my chemical levels guide.
• UV is a useful extra, but it isn’t a replacement for a sanitiser.
• Shower before you get in, and keep a cover on it when it’s not in use.

A top tip for when you drain it: if you let the chlorine drop to pretty much zero first, you can use the water on the garden. Just not while the chlorine is still high, and check your local rules on where pool water can go.

Cold water safety: cold shock

This is the part I most want you to read. I’m a hot tub and pool designer, not a doctor, so everything in this section comes from the organisations that know best.

The RNLI defines anything below 15°C as cold water, and that’s exactly the range a plunge is designed for. When you go in:

• The blood vessels in your skin clamp down, so your heart has to work harder, and your heart rate and blood pressure jump.
• You get an involuntary gasp, and your breathing can speed up dramatically. If your head is under, that’s how people inhale water.
• The RNLI says the worst of it passes in under a minute. The US National Weather Service says the gasping and hyperventilation can last for the first two to three minutes, and that the sudden spike in heart rate and blood pressure can cause heart failure and stroke in vulnerable people.

So, some rules for your plunge:

1. Check with your doctor first if you have heart disease, high blood pressure, diabetes, poor circulation, or you’re pregnant. The Cleveland Clinic lists these as reasons to get medical advice before you start.
2. Never plunge alone, and never after alcohol.
3. Lower yourself in slowly using the step and handrail. Don’t jump in, and keep your head above the water until your breathing settles.
4. Start gently. Cleveland Clinic suggests beginners start at 10–15°C (50–59°F) for one to two minutes, keep sessions to five minutes or less, and avoid water below 4°C (40°F).
5. Get out if you feel lightheaded or anything beyond just being cold. Then dry off and warm up gradually.
6. Use a thermometer. Don’t guess the temperature.
7. Make it childproof. A lockable, rigid cover is a must if children are around. A small, deep pool of very cold water is a real hazard for a child.

Build safety into the design too: handrails or steps for getting in and out, a non-slip surround and good lighting if you’ll use it in the evening. My hot tub steps guide has some ideas.

What about the health benefits?

You’ll see all sorts of claims online, from weight loss to better digestion. I’m not qualified to judge them, so here’s what the medical sources say. The Mayo Clinic Health System says cold-water immersion helps reduce exercise-induced muscle soreness and inflammation, but that researchers are still working out the best way to do it, and regular plunging straight after strength training may blunt some of your gains. Cleveland Clinic describes the evidence for mood, stress and sleep as promising, and for immune benefits as preliminary. Enjoy it for how it makes you feel, and treat big health claims with caution.

Permits and planning

Rules vary by country, state and even town, so check before you build. You don’t want a fine, or worse, to be told to take it out. In England, a garden hot tub usually doesn’t need planning permission unless you’re in a listed building or conservation area, and a small plunge is usually treated the same way. In the USA, in-ground pools and spas often need a permit, and many areas require a barrier or safety cover for water over a certain depth. My guide Do I need a permit for my hot tub? covers where to start.

Cost it out before you start

Add up everything before you dig: materials for the structure and waterproofing, tiles and coping, the chiller, pump, filter and plumbing, the electrics, any permits and labour, and your running costs (power for the chiller and pump, chemicals, filters). You don’t want to be halfway through and run out of cash. A small, well-insulated, covered plunge in the shade is the cheapest one to run.

Common mistakes

• Building it too big. Every extra litre is more to chill.
• Skipping insulation and a cover, then wondering why the chiller never stops.
• Undersizing the chiller for a sunny spot or a warm climate.
• Boxing the chiller in without airflow.
• No filtration or sanitiser, so the water turns in a week.
• A single main drain instead of two with anti-entrapment covers.
• No handrail or step, on a slippery surround.

Want it drawn up for you?

A cold plunge is a custom job, so it’s a great fit for my design service. Draw your idea in the free Tub & Pool Designer Pro software and I can turn it into architectural CAD and a 3D plumbing design, with the chiller, filter and drains laid out properly. If you’re building a hot tub alongside it, my DIY hot tub plans give you the tub side in full, and there’s more on choosing between them in Hot tub vs plunge pool.

FAQs

What temperature should a cold plunge be?

Beginners are usually advised to start at around 10–15°C (50–59°F). Cleveland Clinic advises against going below 4°C (40°F).

How long should you stay in a cold plunge?

Start with one to two minutes and build up slowly. Cleveland Clinic recommends keeping sessions to five minutes or less.

How deep should a DIY cold plunge be?

Typically 0.9–1.5m (3–5ft), deep enough to get your shoulders under while standing or lying down.

Do I need a filter on a cold plunge?

If you want to reuse the water, yes. A pump, a filter and a sanitiser keep it clean for months instead of days.

Can I use a heat pump to chill a cold plunge?

Only if it’s a reversible model with a cooling mode that reaches the temperature you want. Check the spec sheet. Otherwise use a dedicated chiller.

Is a cold plunge dangerous?

It can be. Cold shock raises your heart rate and blood pressure and causes a gasp reflex. Get medical advice first if you have heart or blood pressure problems, never plunge alone, and keep children away with a locked cover.

Planning a cold plunge? Tell me about it 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