Replacing a Shop-Bought Hot Tub with a DIY Concrete Tub: Rob’s Build in France

Last updated: September 2026

“Can I build a new hot tub in the hole where my old one used to be?” It’s a question I get asked a lot, and Rob’s build in France is the perfect answer. He swapped a leaky, hard-to-fix shop-bought hot tub for a concrete one, built into the same space, with the equipment in its own room next door.

How it started

Rob got in touch late in 2021 through the website. Here are some extracts from the email he sent me:

“I have seen your videos and website and wonder if you can help me with a project in France.

Firstly there is a bit of background. About 5 years ago I bought an expensive above ground hot tub. This has been a big disappointment. I am forever mending the thing and fixing leaks (everything is really difficult to get to), to the point that I am completely fed up with it. Last year I built an in-ground swimming pool with a travertine terrace around it, I put the spa half in ground with about a 600mm access all around it. My plan was to cover the access with some sort of wooden structure. Anyway recently I’ve made my mind up that the hot tub needs to go and I shall build a concrete one instead. This needs to fit in the existing hole and I can house all the pumps, filters in a room next to the tub, so it will be so much easier to fix things.

Some of the parts on the existing hot tub are OK and I plan to use these. There are 3 x WP400 pumps and a blower. There may be other parts but I won’t really know until I cut the thing up. The controller and panel are Balboa and may be reusable, but at the moment it’s showing a dry heater warning (the heater is not dry) so I don’t know.

So my questions are… Is this possible? Is this something you could help with, with plans etc? I live in the middle of nowhere in France and my French is terrible. This makes getting parts difficult. Do you supply parts?”

That gives you a real insight into the start of the project. “Of course I can help,” was my response. It’s what I do.

Rob’s reasons are ones I hear all the time. On a shop-bought tub, the pumps and plumbing are packed inside the cabinet, often surrounded by foam, so every repair is a battle. Putting the equipment in a separate, accessible room is one of the big advantages of building your own. I’ve written about that in Hot tub control room design considerations, and about the wider pros and cons in Pros and cons of building a hot tub.

The location

The first step was for Rob to send me some photos of the location and the hot tub he was planning to get rid of. You can see what a great spot it is.

Converting a hole in the ground

This whole project revolves around reusing the hole the previous hot tub sat in. With a project like this, the first thing to remember is that the pipework takes up space.

That means the new hot tub will be smaller inside than the hole you have. Normally, you’ll also be building a new inner skin for the tub, and it’s worth leaving room for insulation too (see DIY hot tub insulation). For the options on building those walls, have a look at Hot tub wall construction: what are the options?

Rob knew what he wanted to achieve. On custom projects, clients usually send me some sketches of what they want to build. Here are Rob’s.

From the sketch and the photos, the next step was for me to come up with the design. I’m not going to post all the drawings, but Rob got a 3D render, a 3D plumbing layout, an architectural drawing and a voiced-over screencast of the plumbing diagram, where I explain what goes where and why. That’s the standard package for a custom design project like this. You can read more about it in Custom designed hot tub: the process explained.

Project paused for a few months

I get to speak to a lot of different people doing what I do, and it’s great. Some people are in a rush to get their project finished, others take their time, and some, like Rob, know from the outset that it’s going to be a long-term project.

He made that clear from the start. We started talking in December 2021, the plans were finalised in June 2022, and he took delivery of the parts in July 2022.

While the winter did its thing, Rob got back in touch in spring 2022. He’d removed the hot tub and stripped out the parts, so he had the final dimensions of the hole, and I could crack on and finish the design.

Recycling some of the parts

Salvaging parts from an existing hot tub can be a good way to keep the cost down. Pumps last an awfully long time, so they’re usually reusable. After inspecting the spa pack, Rob decided to replace it, but to reuse the jet pump, circulation pump, blower and possibly the UV system too.

His WP400 pumps are LX 4HP spa pumps, and they’re still widely sold in 2026 (for example at Hydrospares), so spares and replacements are easy to find. As for the controller, a Balboa “dry heater” message means the system thinks there may not be enough water flowing through the heater. Balboa’s fault code list says to check the water level, flow restrictions, closed valves, air bubbles and pump prime. If it keeps appearing when you know the heater is full, a sensor or board fault is possible, and that’s one reason replacing an old spa pack is often the sensible call on a rebuild.

If you’re weighing up the same decision, these will help: Should I repair or replace my hot tub pump?, How to select a replacement spa pack and Restoring or repairing a hot tub or spa.

Here’s what Rob salvaged. We didn’t use the controller, but we made use of the other parts.

“Hot tub for sale, one careful owner”

And of course, I know you all want to see what Rob did with his old plastic shell tub. Ouch! “Hot tub for sale, one careful owner” was the message he sent me.

Let the building commence

Here’s Rob’s build in his own videos.

For the thinking behind the pipe runs you can see in these videos, see my DIY hot tub plumbing guide.

Leak testing time

Now the fun really started for Rob. It was leak testing time. Did he have a leak or not? I’ll let you watch the video below and find out. As you can see, he tested with unfinished surfaces, which is totally fine. In fact, it’s the right time to do it, because it’s far easier to fix a leak before the tiles go on.

For a DIYer, it’s much easier to test with a full tub than with a pressure test, which is hard to do without the right kit. Rob filled the tub and ran the equipment to see what leaked and what didn’t, so he could fix anything before moving on to the finished surface.

If you find a leak on your own build, my guide How to detect and fix a hot tub leak walks you through tracking it down. Nine times out of ten, it’s in the plumbing rather than the structure.

And he’s finished!

What a result for Rob and his DIY hot tub in France. It’s a truly inspirational build, and Rob should be very proud of himself.

This was one of those builds where I just went “wow” when he sent me the final reveal. I really enjoyed working with Rob over the year or so it took, and I’d like to thank him for all the videos and photos he sent for this case study and my social media. Rob, I really appreciate it. Thank you, and superb hot tub!

Replacing your own hot tub?

If you’ve got a hole where a shop-bought tub used to be, or you’re fed up with a tub you can’t get into to fix, you can do what Rob did. If a standard size fits your space, my DIY hot tub plans come in imperial and metric, with CAD drawings, the plumbing layout, a parts list and the video course. For the construction itself, start with my DIY in-ground hot tub build guide.

If you need to fit an existing hole, like Rob did, sketch it out in the free Tub & Pool Designer Pro software. I can then turn it into architectural CAD and a 3D plumbing design, or do a full custom hot tub design.

FAQs

Can I build a new hot tub in the hole where my old one was?

Yes. That’s exactly what Rob did. Remember that the new walls, pipework and insulation take up space, so the new tub will be smaller inside than the hole, and measure it once the old tub is out.

Can I reuse the pumps from an old hot tub?

Often, yes. Pumps last a long time, and common models like the LX WP400 are still sold, so spares are easy to find. Check the voltage, horsepower and union sizes suit your new design. Controllers are a closer call, and an old spa pack showing faults is often worth replacing.

How do I leak test a DIY hot tub?

The simplest way for a DIYer is to fill the tub before you tile, run all the equipment and watch the water level and every joint. Fix anything you find, then move on to the finished surface.

Are you replacing a hot tub you’ve fallen out of love with? 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