Trey’s DIY Hot Tub Build in Germany: Rebuilding a Historic Cocktail Pool (Case Study)

Last updated: September 2026

It always surprises me how much back and forth I have with customers when I come to put these case studies together, especially when I’m writing one up well over a year after the first contact. With Trey, there must be well over 150 emails and a huge number of photos. Some are of parts and how to fit them, so I’ve left those out, but I’ve included as many as I can of the pictures Trey very kindly sent me.

Trey, who lives in Germany, got in touch through the website, using the regular DIY form you can find at the bottom of this page.

First contact

On 24 April 2022, here’s what Trey sent me:

Hello. We are living in Germany and need to renovate an old in-ground cocktail/spool which is probably 100 years old. It’s historic;so we need to keep the same design/dimensions, but I think the entire structure has to be rebuilt. As I rebuild it, I would like to add in jets, pump, heater, etc. Fortunately there is a covered area at pool level about 10 feet away from the pool. Please let me know what a design from you might cost? The pool is circular with a 12 foot diameter at a 3 foot depth. There is a set of stairs outside this diameter descending into the pool.

This sounded interesting from the start, and definitely something I thought I could help with. So I got back to Trey, and here’s what came back:

Attached are some pictures of the pool in November of last year. It was completely filled in with earth when we bought our house the year prior and I have excavated it to expose the interior. It does have a working drain.

Would ask your recommendation as to whether the current structure is completely or partially salvageable. The floor has hairline cracks. The walls have more significant cracks and the ring around the top of the pool is more or less completely separated from the pool walls. The stairs have significant cracks and need to be replaced (just my assessment). I’m not sure what is going on with what I have termed the “two teeth” you can see in the picture. technically I should keep them in any renovated design, but I’m also ok with removing them.

I’m wondering if filling in the cracks in the slab and/or walls and then tiling or epoxy painting would last 5-10 years. The slab has likely settled as much as possible over the last 50-100 years, but I currently have no idea whether or not the builder used any kind of rebar during construction. There is some structural steel in the stairwell mentioned below. Small I beams. Not sure they had rebar back then. So I worry about the wall integrity and ability to withstand the water pressure from the filled pool.

The pool does have a working drain pipe connected to the house’s black water drain system. At least that’s a plus.

And there is a covered structure next to the pool, about 3 meters away which goes down into the earth about 12 meters and is a staircase with four levels. You can see it in the picture at the top of the pool stairs and at the end of the short walkway. It looks like an elevated patio and has the round picnic table on top of it. I can assure you that the entire structure goes down into the earth 12+ meters. I can put the pool control room on the top level of the structure and at the same height as the pool, whether that be a drain height or skimmer height, drain height or even lower. There is a natural gas line nearby as well as electricity available to this area

After viewing some of your videos, I think I would excavate 30+ cm around the entire tub so as to assess the pool wall from both sides and to install the plumbing. If a retaining wall is needed, I’d have to excavate more in order to build that. I’d ask your recommendation on the need for the retaining wall as there are plenty of tree roots around the pool currently; so minimal excavation would be nice. But also want to take into consideration the need to maintain the plumbing.

Your price for design seems fair, especially if you are available for telephone consult during the construction process. I am a DIY person and will definitely do any excavation myself as well as plumbing. Might also install the pump and heater myself but have a professional hook them up to power and/or gas. Also I am a degreed structural/civil engineer, although have not practiced enough to trust any design I might come up with as far as rebar, etc.

Here are the photos Trey sent me. As you can see, the pool had clearly seen better days.

Repair or rebuild?

Our first discussions were about whether we could salvage and repair the walls. The conclusion was a resounding no. There was no real way of telling how sound they actually were, or whether they’d last. Given their age, the chances of them surviving being cored for jets were slim to none. The base was also questionable, as you can see from the cracks in the floor.

So the decision was made that it had to go. We’d design something new in keeping with the size and shape of the original, just built with modern materials and methods that we know work. If you’ve got an old structure of your own, my guide to restoring or repairing a hot tub or spa is a good place to start.

The design

I set to work on the design and tried to keep it in context with what was there already. As with all these kinds of projects, the plan was pretty fluid, and we ended up with more of a “keyhole” layout than we’d originally planned. If a small pool you can also heat like a hot tub appeals to you, have a look at What is a spool? and how to build a DIY cocktail pool or spool.

Building the walls

Trey’s first step was to clear the area and lay a new foundation for the blocks. His plan was to get the outer blocks in place, then build the seats, and finally pour the floor once the seats and walls were in.

Trey originally wanted to use ICF block, but given the shape of the pool, it was going to be difficult to source. So he went with CMU block and added his own insulation, as you can see below. (For the pros and cons of each, see hot tub wall construction: what are the options? and cinder block hot tubs.)

You can see how it progressed in the photos below.

As you can see, Trey is pretty handy, and the build progressed really well. The engineer in Trey came out clearly too. Some of the drawings and ideas he sent me were so well put together, and it made working with him very easy indeed.

Waterproofing and finishing

By late May 2023, with the walls and plumbing in place, it was time to turn to the surface of the tub. The next stages were the skim coat, then sealing the skim coat with a PVA wash, followed by a waterproofing layer. I believe Trey used an Ardex S7 product or similar. I can’t find our discussion about this part of the project, but it looks similar. There’s more on the materials in waterproofing, grout and tile adhesive.

He also built the exterior with aged blocks, again to keep it in keeping with the pool that had to be removed. Trey’s build looked good from the outset, but now it really started to come together, and you can see shades of what the final hot tub will look like.

The control room

By late July and early August 2023, Trey was working on the control room and the tiles before he stopped for his summer holiday.

I haven’t said much about the plumbing so far, so here’s a good opportunity. Trey has a dual-pump system: a circulation pump for heating (by air source heat pump) and filtration, and a separate jet pump for the jets. This is a pretty standard layout now for anyone using an external heater like this. See how many pumps you need for a DIY hot tub.

What isn’t standard is that Trey has put all the kit in the basement of his house. That’s great in terms of keeping the kit covered and out of the elements. (I’ve written more about control room design.)

Of all the control rooms I’ve seen over the years, this has to be one of the best-looking any customer has done! The eagle-eyed among you will have spotted the Comfortline air source heat pump, which is the model I use in Europe (here’s my Comfortline heat pump review). To the right, you can see an opening into the basement, so there’s plenty of airflow for the heat pump to work properly. If you’re adding one yourself, read my ultimate guide to air source heat pumps for hot tubs and how to choose the right air source heat pump.

By September 2023, this is where things stood. I have to admit, it’s one fantastic-looking hot tub.

Filling and testing: leaks and an airlock

Then came a challenging week for Trey. First, he had to fix some leaks on the suction side, which meant digging back to the pipes and re-sealing them. It’s very frustrating work when you’ve already done the job once. (My guide to finding and fixing a hot tub leak is worth a read, and pressure test before you bury any pipe.)

With the leaks sealed, it was on to testing the electrical system. The heating and filtration worked first time, no problem. The jets were another story. They’re on their own separate system, with their own suction and their own jet pump, and while Trey was filling the tub, air got trapped in it.

It was a huge airlock, and it was quite a challenge to remove. It was so big that the jets weren’t putting out any pressure at all.

When you fill your hot tub, it’s always a good idea to fill through the filter or from the lowest point in the system as much as you can, and let it overflow. This helps avoid big airlocks like Trey’s. There’s more in how to fill a hot tub the correct way and Help, there’s an airlock in my hot tub!

Watch the video below for the update.

Latest update

Restoring or rebuilding something unusual?

Projects like Trey’s, where you’re working around an existing structure, a historic shape or an unusual equipment location, are exactly where a proper design pays off. Sketch your idea in the free Tub & Pool Designer Pro software, and I can turn it into architectural CAD and a 3D plumbing design. If a standard size suits you, my DIY hot tub plans include the CAD drawings, plumbing layout, parts list and video course, and there are plunge and cocktail pool plans too.

FAQs

Can you repair a very old concrete pool instead of rebuilding it?

Sometimes, but not if you can’t tell how sound the structure is. In Trey’s case the walls were cracked, the top ring had separated and nobody knew whether there was any rebar, so coring the walls for jets wasn’t worth the risk. We rebuilt it.

Can the control room be in a basement?

Yes. Trey’s is. It keeps the kit dry and out of the weather, but an air source heat pump needs plenty of airflow, which is why there’s a large opening in Trey’s basement next to it.

Why are my jets weak after filling a new hot tub?

Usually trapped air. Fill through the filter or from the lowest point, let it overflow, and bleed the air from the pump before running the jets.

Why use two pumps?

A small circulation pump handles filtration and heating quietly and cheaply, and a separate jet pump only runs when you want the massage jets.

What do you think of Trey’s rebuild? Let me know in the comments below. I read them all.

Cheers,
Andi

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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