Built-In Spas: How to Design and Build Your Own In-Ground Hot Tub

Last updated: September 2026

Have you ever dreamt of your own private spa, somewhere the stress just melts away? That dream became a reality for me when I decided to build a built-in spa at home. Little did I know that one project would end up becoming the business it is today.

A built-in spa is a permanent hot tub that’s part of your garden, deck, pool or even your house, rather than a plastic shell sitting on a slab. In this post I’ll explain what they are, where they work, how they’re built, and what you need to think about for the structure, plumbing, heating, electrics and safety.

What are built-in spas?

Built-in spas, also called in-ground spas or custom spas, are permanent installations designed into your home or outdoor space. Unlike a portable hot tub, which is a finished product you drop into place, a built-in spa is constructed on site, so it can be any size or shape and can blend into the surroundings rather than sitting on top of them.

They’re usually built from concrete, block or ICF, then waterproofed and finished with tiles. Inside, they have the same kit as any hot tub: jets, seats, lights, a heater, filtration and controls. Some add waterfalls, spillways or a swim jet.

Where can a built-in spa go?

Location	What to think about
In-ground in the garden	The classic. Dig depth, groundwater (fit a hydrostatic relief valve), and where the spoil goes. See in-ground hot tubs: a DIYer’s guide.
Semi-raised or raised	Less digging, but the walls hold the full water pressure with nothing behind them. See above-ground hot tubs.
Built into a deck	The tub needs its own foundation, not the deck frame. See hot tub deck ideas.
Beside or spilling into a pool	Decide whether the water is shared or separate. See the hot tub pool combo guide, the DIY plunge pool guide, the cocktail pool and spool guide and adding a spillover hot tub to a pool.
Indoors or in a basement	Structural loads, drainage, and above all humidity. See can you build a hot tub in a basement?

One big advantage of a built-in spa is that it can be designed for year-round use, with proper insulation, efficient heating and a cover that fits perfectly. For a relaxing soak, stress relief or easing aching muscles, it’s there whenever you want it, with no travel or membership fees.

Can I build my own built-in spa?

Absolutely, and I can 100% help you do it. Here’s what to weigh up first:

1. Skills. You need a reasonable grasp of basic construction and plumbing. If you’ve done some DIY and are willing to learn, you’re in a good place. I’d never laid a block before my build.
2. Permits. In England, a garden hot tub usually doesn’t need planning permission unless it’s at a listed building or in a conservation area, but the electrics must meet Building Regulations. In the USA, many cities and counties want a permit for an in-ground spa. See Do I need a permit for my hot tub?
3. Design. Plan the size, seats, access to power and water, drainage, the control room and how it fits with the house or garden. This is what my plans and design service are for (more below).
4. Electrics. The supply must be installed by a qualified electrician. More below.
5. Time and budget. A built-in spa is a real project. Be realistic. See how long does a DIY hot tub build take?.

Construction options

	Poured concrete	Concrete block	ICF
How it’s built	Rebar cage inside formwork, then concrete poured	Hollow blocks laid, reinforced and core-filled	Foam blocks stacked, rebar added, then filled with concrete
Shapes	Any shape you can build a form for	Best for squares and rectangles	Mostly straight walls
Insulation	Poor, add it	Poor, add it	Built in
Cost	Formwork adds cost and time	Cheapest materials	Higher block cost
DIY-friendly?	With help. The formwork must hold the pour.	Yes, if you’re patient	Yes, the easiest

• Concrete is durable and can be shaped however you like, including built-in seats, steps and contours. Proper reinforcement matters: a crack means a leak.
• Block is cheap and strong once it’s reinforced and core-filled. Mine is block, and it’s perfectly good. Concrete block isn’t an insulator, though, so an in-ground block tub loses heat into the ground unless you add insulation.
• ICF gives you a reinforced concrete wall with insulation on both sides in one go. It’s light, quick to build and easy to route plumbing through. If I built again, I’d use ICF. See the best ICF blocks for a hot tub.

Whatever the walls, the waterproofing is what keeps the water in. That means render, a pool-rated waterproofing membrane in two coats, and a flood test before tiling. Then tile with pool-grade adhesive and grout. A painted-on sealer over bare concrete isn’t a waterproofing system. See waterproofing, grout and tile adhesive and the full comparison in block, ICF or poured concrete?

Plumbing considerations

1. Layout and design

• Design a layout that suits the spa’s size, shape and features, with short, direct runs back to the control room.
• Plan circulation so there are no dead spots.
• Put valves and unions where you can reach them. See DIY hot tub plumbing.

2. Jets

• Choose jet types and positions for backs, shoulders and feet at each seat. See DIY hot tub jets.
• Balance the jets against the pump. Too many jets on one pump and they’re all weak. As a rough guide, one jet pump runs up to about 16 jets. See how many pumps do you need?
• Add air with a blower or venturi if you want bubbles. See air blower or venturi?

3. Drains and skimmer

• Fit two main drains per suction line, at least 3ft (1m) apart, with anti-entrapment covers. In the USA, the VGB Act requires ANSI/APSP/ICC-16 drain covers. See installing a main drain.
• A skimmer keeps the surface clean and sets the water level. See how to install a skimmer.
• Fit a hydrostatic relief valve in an in-ground spa.

4. Filtration

• Cartridge filters are the usual choice on a hot tub. They’re easy to clean and good at catching fine debris.
• Sand filters can work, but they need the right flow and backwashing. See can I use a sand filter on a DIY hot tub?
• DE filters give the finest filtration but are more fiddly.
• Size the filter generously and clean it regularly. See cartridge or sand filter?

5. Pumps

• Match the jet pumps to the number of jets and the length of the pipe run.
• A small, quiet circulation pump for filtration and heating saves running the big jet pumps for hours.
• Variable-speed pumps save energy, but check they’ll work with your spa pack first. See can I use a variable speed pump in a hot tub?

6. Heating

• Electric heater in the spa pack: simple and reliable, but every kW of heat costs a kW of electricity, and it’s slow on a big tub.
• Air source heat pump: the cheapest to run. The US DOE puts pool heat pumps at a COP of 3 to 7. I cut my own running costs by 66% when I fitted one. See how I cut my hot tub running costs by 66% and the guide to air source heat pumps for hot tubs.
• Gas or propane: the fastest heat-up, but more expensive to run.
• Whatever you use, keep it at or below 40°C (104°F). That’s the CPSC’s maximum.

7. Draining and winterising

• Build in a way to empty the spa: a drain valve at the lowest point of the plumbing, or a submersible pump.
• Plan for frost. Either keep it running through the winter or drain and blow out the lines. See winterizing a hot tub.

8. Access and maintenance

• Fit shut-off valves either side of the pumps, heater and filter so each can be removed without draining the tub.
• Make the control room bigger than you think, and give it a drain. I learned that one the hard way. See control room design considerations.

9. Water treatment

• Chlorine, bromine or salt chlorination are the main choices. UV-C can reduce chemical use but doesn’t replace a residual sanitiser, and I don’t think ozone is worth it. See why you don’t need ozone.
• The US CDC recommends at least 3ppm free chlorine in hot tubs, pH 7.0–7.8, and no cyanuric acid. The full ranges are in my hot tub chemical levels guide.

Electrics

This is the one part to hand over. In the UK, a new hot tub circuit is notifiable under Part P, and the IET says outdoor hot tubs should be treated under BS 7671 Section 702, with an isolator outside zone 1, 2m from the edge. In the USA, NEC Article 680 requires GFCI protection, a disconnect within sight of the spa, and bonding. See A guide to hot tub electrics.

Indoor built-in spas

A spa in a bathroom, spa room or basement looks amazing, but warm water in a closed room creates a lot of moisture. Plan for mechanical ventilation and a dehumidifier sized for the room, plus insulation and a vapour control layer, or you’ll get condensation, mould and rot. Check the floor can take the weight, plan a floor drain, and remember Section 702 or NEC 680 applies indoors too.

Safety

• A lockable cover whenever it’s not in use.
• Barriers: in the USA, the CPSC’s barrier guidelines count spas and hot tubs as pools, with a 48in barrier and self-closing, self-latching gates, and some codes accept a compliant safety cover on a spa. Check locally. In the UK there’s no legal requirement, but RLSS UK recommends secure fencing at least 4ft high around home pools.
• Non-slip steps and a handrail. See hot tub steps.
• Temperature no higher than 40°C (104°F), and supervise children.

Common built-in spa mistakes

• Digging to the water depth, so the seats end up too shallow.
• No insulation on block walls.
• Relying on waterproofing additives instead of a membrane, or tiling before a flood test.
• Too many jets for one pump.
• A cramped control room with no drain.
• An indoor spa with no dehumidification.

There are more in What I’d change if I built my DIY hot tub again.

Planning your own built-in spa?

If one of the standard sizes fits, my DIY hot tub plans give you CAD drawings with every dimension, the plumbing layout, a full parts list and the video course. There are also infinity hot tub plans and swim spa plans.

If your space is unusual or you want something truly custom, sketch it in the free Tub & Pool Designer Pro software, and I can turn it into architectural CAD and a 3D plumbing design. See custom hot tubs: what’s possible? for ideas. Building a spa alongside a pool? The hot tub and pool combo design consultancy covers both.

FAQs

What is a built-in spa?

A permanent hot tub built on site, in-ground, into a deck, beside a pool or indoors, usually from concrete, block or ICF and finished with tiles.

Can I build a built-in spa myself?

Yes. The structure, plumbing and tiling are very achievable for a confident DIYer with good plans. Leave the electrics to a qualified electrician.

What’s the best material for a built-in spa?

For most DIY builds, ICF, because it’s strong, easy to build and insulated. Core-filled block is cheaper but needs added insulation.

Is a built-in spa more expensive to run than a portable hot tub?

It doesn’t have to be. Insulate the walls and floor, use a good cover, and heat it with an air source heat pump.

Do I need planning permission for a built-in spa in the UK?

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

Planning a built-in spa? Drop your questions 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