Adding a Spillover Hot Tub to an Existing Pool: What to Plan

Last updated: September 2026

Dreaming of a hot tub that spills over into your swimming pool? It’s one of the most popular upgrades I get asked about, and it can transform a backyard. But you can’t go into a project like this without a plan. You’re joining a new structure to an existing one, and sharing water, plumbing and heat between them.

In this post I’ll go through the key things to think about before you start: whether your pool can take it, the structure, the plumbing and valves, heating, chemistry, electrics and safety. Get these right and you’ll have years of enjoyment. Don’t forget, I have a YouTube channel too.

Is your pool suitable?

Before anything else, look at what you’ve got:

• Where would the spa go? Most spillover spas sit at one end or in a corner, often raised a little above the pool so the water drops into it. Is there space, and is it close to the equipment?
• What’s the pool built from, and what condition is it in? Concrete and block pools are the easiest to add to. Liner and fibreglass pools are harder, because cutting into them to create a spillway can compromise the shell or void a warranty. Talk to the pool’s manufacturer or installer first.
• What equipment does it have? Note the pump, filter and heater sizes. You’ll need them for the plumbing decisions below.
• Design harmony. The spa should look as if it was always meant to be there. Match or complement the coping, tiles and colours, and think about the shape. A square spa on a rectangular pool is far easier to build than a round spa on a kidney pool. Google and Pinterest are great for inspiration.

If you haven’t got the pool yet and you’re planning both from scratch, read my hot tub pool combo guide instead. It covers layouts from the start. For a smaller pool with a spa alongside, see my guides to building a DIY plunge pool and a cocktail pool or spool. For the broader question of adding any kind of hot tub, see Can I add a hot tub to my existing swimming pool?

Structure: give the spa its own foundation

A spa full of water, plus the people in it, is several tonnes. That weight has to go straight down into a proper foundation, not onto the edge of the pool.

• Build the spa on its own reinforced slab or footing, on ground that hasn’t been disturbed, or that has been properly compacted. The ground right next to a pool is often backfill, which can settle.
• Don’t cantilever it off the pool wall or rest it on the deck unless an engineer has designed it that way. The last thing you want is your new hot tub sinking into your pool and cracking both.
• Where the spa meets the pool (the shared wall or the spillway), get a structural engineer to look at how the two join, especially if you’re cutting into the pool’s top beam or coping.
• Build the spa itself like any in-ground hot tub: a reinforced slab, ICF or core-filled block walls with rebar, a pool-rated membrane and a flood test before tiling. See block, ICF or poured concrete? and rebar explained.
• Get the spillway dead level. Any low spot takes all the flow and leaves a dry patch elsewhere.

Plumbing: how a spillover works

In a typical spillover, the pool’s filter pump draws water from the pool, pushes it through the filter and heater, and sends some or all of it back into the spa. The spa fills and spills over into the pool. The spa’s jets run on their own pump, drawing from the spa’s own drains.

Spa mode and pool mode

Think about how you plan to use the pool and the tub. If you want the tub hot while the pool stays cooler, you have a problem: the spillover is constantly topping the tub up with cooler pool water. If you’re trying to heat the hot tub while adding cooler pool water to it, it’s counter-productive.

The answer is two 3-way valves (manual or motorised), one on the suction side and one on the return side:

• Pool mode: the pump draws from the pool’s skimmer and main drains, and returns to the spa and/or the pool. The spa spills over.
• Spa mode: the pump draws from the spa’s drains and returns only to the spa. The spill stops, and the tub circulates and heats on its own.

Motorised valves on a controller can switch between the two with one button. Being able to stop the spill is also handy at night, when the sound of falling water can carry, and when you’re not using the pool at all.

Can the existing system cope?

Pools are designed around a specific amount of water, pump and filter. Adding a spa adds water to be filtered, and a spillway needs flow to look good. Pool designer Skip Phillips recommends about 5 gallons per minute per linear foot of edge for a vanishing edge, more if the edge is rounded rather than sharp. So a 3ft (0.9m) spillway needs roughly 15 GPM (57 litres a minute) or more, on top of the spa’s own needs. That can put the existing pump and filter under pressure if they aren’t sized for it. See how to correctly size a swimming pool overflow or spillover and tips for selecting a new pool pump.

Keep the jets separate

Pool plumbing moves a large volume of water slowly. Hot tub plumbing moves a small volume quickly. So the jets need their own pump, drawing from at least two main drains in the spa with anti-entrapment covers. In the USA, the VGB Act requires ANSI/APSP/ICC-16 drain covers. See why pool and hot tub plumbing should remain separate, how many pumps do you need? and installing a main drain.

Filtration: shared or separate?

• Shared (integrated): the pool’s filter serves both, using the valves above. It’s simpler to maintain, but the filter has to handle the combined volume and the spillway flow.
• Separate: the spa has its own circulation pump and filter as well. That gives you independent control and makes winter use easy, at the cost of more equipment.

If you want to use the tub in winter while the pool is closed, plan for dedicated heating and filtration for the tub, and a way to stop the spill completely.

Heating

There are three common approaches:

• One heater, shared. The pool heater serves both. In spa mode it heats the tub alone, which is quick because it’s a small volume. This is the cheapest to buy, but you’re heating one or the other at full tilt, not both.
• Separate heaters. The pool has its own heater (gas or an air source heat pump), and the spa has its own electric heater or heat pump. It’s the simplest to control, and essential if you’ll use the tub while the pool’s closed.
• Stop the spill while heating. Whichever you choose, heating is far more efficient with the spill turned off.

An air source heat pump is the cheapest way to heat the pool side. The US DOE puts pool heat pumps at a COP of 3 to 7. Covers matter too. The DOE says a pool cover can cut heating costs by 50–70%, and a spillover that’s running loses heat as the water falls. Keep the spa at or below the CPSC’s 104°F (40°C) maximum. See how to choose the right air source heat pump and heater size for your pool or hot tub.

Chemistry with shared water

Hot tubs need more sanitiser than pools. The US CDC recommends at least 1ppm free chlorine in pools (2ppm with stabiliser) and at least 3ppm in hot tubs, with pH 7.0–7.8, and no cyanuric acid in hot tubs. With a spillover you run one set of water, so when the tub is hot and in use in spa mode, treat it as a hot tub: bring the chlorine up to hot tub levels, and keep stabiliser as low as you can in the shared water. See cyanuric acid in hot tubs and my hot tub chemical levels guide.

Electrics and bonding

Leave the electrics to a qualified electrician, and tell them it’s being joined to an existing pool.

• USA: NEC Article 680 applies, with GFCI protection and equipotential bonding. That covers the steel in the structure and paving and metal parts near the water, connected with solid copper no smaller than 8 AWG. The new spa’s rebar has to be bonded before the concrete goes in, and connected to the pool’s existing bonding. See ground bonding explained.
• UK: BS 7671 Section 702 applies, and the IET says outdoor hot tubs should be treated under it too, with an isolator outside zone 1, 2m from the edge. The work is notifiable under Part P.

More in A guide to hot tub electrics.

Safety, barriers and permits

• Barriers (USA): the CPSC’s barrier guidelines count spas and hot tubs as pools. Your existing barrier must enclose the new spa too: at least 48in high, with no gap a 4in sphere can pass through and self-closing, self-latching gates. Local codes set the rules.
• Barriers (UK): no law requires one for a private pool, but RLSS UK recommends secure fencing at least 4ft high.
• Covers: a lockable cover on the spa, and ideally a cover on the pool.
• Steps and edges: non-slip, with a handrail into the spa. A raised spa wall between spa and pool invites people to jump or dive from it. Don’t.
• Permits: in the USA, adding a spa to a pool usually needs a building permit and electrical inspection. In England, a garden pool or hot tub often doesn’t need planning permission, but check if you’re at a listed building or in a conservation area. See Do I need a permit?

Maintenance

Owning a spillover spa and a pool means committing to maintenance for both. Test more often when the spa’s in use, clean the spa filter more often than the pool filter, and keep the spillway edge clean so it flows evenly. My beginner’s guide to hot tub maintenance covers the spa side.

Common mistakes

• No way to isolate the spa, so it can never get properly hot.
• Running the jets off the pool pump.
• A spa built on backfill or hung off the pool wall.
• A spillway that isn’t level.
• Undersized pump or filter for the extra flow.
• Forgetting to extend the barrier and the bonding.

Real builds

For inspiration, see How to add a hot tub to an existing pool (case study), Scott’s hot tub added to his pool in Arizona and Danny’s pool and spillover hot tub combo.

Want help with the plumbing?

Getting the valves, pumps and heating right is what makes a spillover work well. If your pool is already built, my hot tub and pool combo plumbing consultancy ($2,497) gives you 3D plumbing schematics for both systems, showing jets, pipe runs, pumps, heaters, filters and valve positions, plus a parts specification and quotation and email support until both are running. It doesn’t include structural design or CAD drawings.

If you need the spa designed as well, the custom hot tub and pool combo design consultancy ($2,997) covers the design too. Or sketch it yourself in the free Tub & Pool Designer Pro software, and I can turn it into architectural CAD and a 3D plumbing design.

FAQs

Can I add a spillover hot tub to my existing pool?

Usually, yes, especially with a concrete or block pool. The spa needs its own foundation, and the plumbing has to be designed so the spa can be isolated. Check with the manufacturer first if you have a liner or fibreglass pool.

Can the hot tub be hotter than the pool?

Yes, with valves that switch to spa mode. The spa then draws from and returns to itself, the spill stops, and it heats on its own.

Can the pool pump run the hot tub jets?

Not well. Pool pumps are designed for slow, high-volume flow. The jets need their own pump.

How much flow does a spillway need?

As a starting point, about 5 GPM per foot of edge (roughly 60 litres a minute per metre) over a sharp, level edge. Rounded or uneven edges need more.

Does the spa need a fence too?

In the USA, spas count as pools for barrier rules, so the spa must be inside a compliant barrier. In the UK it’s not law, but a lockable cover and a fence are strongly recommended.

Adding a spa to your pool? Tell me about it 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