Trey’s DIY Hot Tub Build – Germany

Trey's DIY Hot Tub Build

It always surprises me just how much communication back and forth I have with customers when I come to put these case studies together. Even more so when I am starting to write it up well over a year since the initial contact. I must have well over 150 emails and so many photos back and forth. Some are of parts and how to fit them so I am not going to include those in this case study. But I will include as many of the pics that Trey very kindly has sent me.

Trey, US services Veteran now living in Germany got in touch with me through the website, on the regular DIY Form that you can find at the bottom of this page.

24th of April, 2022 – here is what he 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 that I thought I could help with. So I reached out to Trey. Here is 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 phots that Trey sent me – as you can see, the pool had clearly seen better days.


The initial discussions that Trey and I had were on wether we thought that we could salvage and repair the walls. The conclusion was a resounding no. There was just no real way of determining how good they actually were and wether they were going to last. Also, given the age of them, the chances of them surviving being cored for jets was slim to none. The base was also questionable as you can see the cracks already on the floor.

Unfortunately, the decision was made that it needed to go and we would re-design something new in fitting with the size and shape of the original – just made with modern materials and methods that we know will work.

I set to work on the design and tried to keep in context with what was there already. As with all these kinds of projects, the plan was pretty fluid and we ended up going for more of a “key hole” design and layout than we originally planned for.

You can also find me on my socials;

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

Trey was originally looking to use ICF block, but given the shape of the pool, it was going to be difficult to source so he decided he would go with CMU block and then add his own insulation as you can see below.

You can see just how this progressed in the photographs below.

As you can see, Trey is pretty handy in terms of the DIY here and the build is progressing really well. The engineer in Trey also came out very clearly as some of the drawings and ideas he sent me were just so well put together. It has made working with Trey very easy indeed!

We’re now in late May 2023 and with the walls in place and the plumbing in situ too, it was time to move the attention to the surface of the tub. The next stages for Trey were the skim coat, then the sealing of the skim coat with a PVA wash, followed by a waterproofing layer. I believe Trey went for an Ardex S7 product (or similar, I can’t find our discussions on this part of the project but it does look similar).

He also put in place the exterior with the aged blocks – again, the idea being to keep in fitting with the pool that had to be removed from that location. Whilst Trey’s build was looking good from the outset, things started to really look good now and you can see shades of what the final hot tub is going to look like.

Fast Forward to late July, early August 2023 and Trey was working on the Control Room as well as the tiles on the tub before he stopped for his summer vacation.

I’ve not spoken too much about the plumbing setup so far so here is a good opportunity to do so. Trey is having a dual pump system, circulation pump for the heating, which will be done by air source heat pump, and also filtration, then a separate jet pump for the jets. This is a pretty standard kind of a layout now for anyone that is using an external heater such as this.

What is not “standard” for this is that Trey is locating all of the kit in the basement of his house. This is great in terms of the kit needing to be covered and out of the elements.

In terms of control rooms that I have seen over the years, this has to be one of the best looking I’ve ever seen a customer do! Those eagle-eyed amongst you will have noticed the Comfortline Air Source which is the model that I use in Europe. Also, to the right you can see that there is an opening into the basement so there is plenty of air flow to make sure the air source will work just fine.

And here we are, September 2023 and this is the current state of play. I have to admit, this is one fantastic looking hot tub. Can’t wait to share the next update with you. Should be full of water and being tested very soon!


This week has been a challenging week for Trey and his DIY hot to build. Firstly, he had to fix some leaks that he had on the inward suction which involved digging back the pipes and then re-sealing; it’s very frustrating work when you’ve already done the job once.

Then, with the leaks all sealed up, it was then on to testing the electrical system. The heating and filtration system worked just fine first time no problem. However, Trey did run into a problem with the Jets. The Jets sit on their own separate systems.

They have their own inward suction and they also have their own jet pump. What happened was whilst Trey was filling the hot tub, he actually managed to get some air trapped in the system.

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

When filling 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 to overflow. This just helps to avoid bigger locks as we’ve seen with Trey.

Watch the video below for the update.

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 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.

Today, I've helped over 1000 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

DIY Hot Tub / Swim Spa Case Study – Darren, AZ, USA

Darren got in touch with me April 2024 because like most of my customers, he [...]

How Noisy is an Air Source Heat Pump?

In my latest video, which you can find below, I take a look at how [...]

Installing a Main Drain for Your Hot Tub or Plunge Pool

Installing the main drains that you are going to need on your hot tub or [...]

Rebar for Hot Tubs and Pool Construction Explained

Imagine soaking in a steaming hot tub or cooling off in a refreshing pool on [...]

Infinity Hot Tub with Swim Jet Takes Shape in Virginia

Have you ever dreamt of soaking in a luxurious hot tub while enjoying a mesmerizing [...]

Keeping Your Pool Crystal Clear: A Deep Dive into Sand Filters

Sparkling pool water is every pool owner's dream. But achieving this oasis of refreshment requires [...]

Demystifying Spa Pack Wiring Diagrams: A Comprehensive Guide with Video Insights

Have you ever stared at a spa pack wiring diagram, feeling utterly lost? You're not [...]

Can I Heat a Hot Tub with Solar Power?

Imagine stepping into a steaming hot tub on a chilly evening, surrounded by nature, and [...]