How to Build a Composting Toilet: The Complete Guide

A practical guide from a team that's been making composting toilet hardware since 2009. Everything you need to build a compost loo that actually works — the urine separator, the plumbing, the chamber, the ventilation, and where the waste goes — illustrated with real customer builds from around the world.

Why build your own composting toilet?

A well-built compost toilet does three things a flush toilet can't: it saves around 25 litres of water per person per day, it turns human waste into valuable compost rather than sewage, and it works completely off-grid with no plumbing, no septic tank and no mains power. For van conversions, tiny houses, allotment sheds, off-grid cabins and remote homesteads, a DIY compost toilet is often the only practical option — and frequently the most satisfying part of the build.

And once you understand the underlying principle, building one is genuinely simple.

The single most important thing to get right

Urine separation. If you take only one thing from this guide, take this: the difference between a compost toilet that smells terrible and one that smells of nothing at all is whether the urine is diverted away from the solids at source.

The unmistakeable compost-loo smell comes from urine and faeces mixing together. Ammonia-producing bacteria thrive in that wet, mixed environment. Keep them apart — and keep the solids dry by covering them with sawdust or wood shavings after each use — and the system works beautifully.

Everything else in this guide is detail. This is the principle.

The three essential components

Every good DIY compost toilet, regardless of style, needs three things:

1. A urine separator — the part that does the diverting
2. A solids chamber — usually a bucket or larger composting container
3. Ventilation — a simple vent pipe to keep the chamber dry and aerated

Plus a seat platform, a finish, and a system for what happens to the urine and solids after they leave the toilet. Here's how it all fits together under the seat:

1. The urine separator

The urine separator is a shaped insert that sits under the toilet seat. It catches urine in a front bowl and diverts it out through a small outlet pipe, while the solids drop straight through into the chamber below.

You can buy one or try to make one. We're biased — we've been making them since 2009 — but here's our honest take: making a separator that catches reliably for both male and female users, in all sitting positions, with a smooth easy-clean surface and a properly sized outlet, is harder than it looks. We've watched a lot of DIY versions get scrapped after a month of awkward use.

Either way, the key features to look for are:

• A wide, shallow front bowl that catches urine from both seated male and female users
• A smooth, glossy easy-clean surface (rough surfaces hold smell)
• A generous outlet — ours has a tapered 30–35 mm outlet that wedges into standard 1¼" (32 mm) waste pipe without blocking
• Dimensions that fit under a standard toilet seat hole — roughly 170–330 mm wide
• Strong, durable plastic that won't crack or stain over years of use — ours is solid 4 mm ABS

Our Urine Separator is our lightweight original (white or black), and the Complete Urine Separator is the all-in-one design with a built-in metal filter. There's a section on choosing between them further down.

Plumbing: choosing your pipe

Our separators are designed to work with 1¼" push-fit, 1¼" solvent-weld and 32 mm flexible waste pipe. Each has its place:

• Push-fit — easiest to modify later if you want to change your design. A good all-rounder.
• Solvent-weld — the strongest joint, but once it's glued it can't easily be changed.
• Flexible 32 mm — easy to route around tight corners, though the internal ribs can occasionally catch sawdust.

The number that matters is the internal diameter of the pipe. The separator's outlet nozzle tapers from 30 mm to 35 mm, and that taper is deliberate — it lets several pipe types wedge on snugly. If you're using 1¼" push-fit and the fit is tight, simply deburr the inside edge of the pipe with a deburring tool or a twist of sandpaper, or slip on a 1¼" connector, and it pushes home easily.

One golden rule: the outlet pipe must slope downhill all the way to your urine container. Gravity does all the work here — even a slight uphill section will cause backups.

2. The solids chamber

The solids chamber is whatever the solids drop into. For the simplest setups a 25-litre plastic bucket works perfectly — cheap, available everywhere, and easy to lift and empty.

Some popular options:

• 25-litre buckets — the standard DIY choice. Easy to empty, easy to swap, easy to find.
• Wheelie-bin setups — much larger capacity; rat-proof, waterproof, on wheels and cheap.
• Twin-chamber boxes — alternate between two compartments so one composts while the other fills.

The key principle: keep the solids dry. After each use the user adds a scoop of sawdust, wood shavings, coir or leaf mould. This soaks up residual moisture, covers the deposit and starts the composting. Without urine in the mix, this works brilliantly; with urine mixed in, you have a wet anaerobic mess.

3. Ventilation

A simple vent pipe — typically 50–75 mm diameter — running from the chamber up and out through the roof or wall keeps fresh air flowing through the solids chamber. This dries the contents further and stops any residual smell escaping into the room.

For small van or tiny-house installs a passive vent is usually enough. For larger or busier installations (glamping sites, festivals) a small 12 V or mains fan in the vent improves airflow significantly.

Fitting the separator: position, height & cutting to fit

This is the part most people get wrong, so it's worth taking your time.

Position — fill the front third. The separator's bowl should fill roughly the front third of the toilet seat hole. That's the sweet spot where urine reliably enters the bowl for both male and female users when seated, while leaving the solids drop-zone clear behind it.

Two ways to mount it. You can screw the separator directly to the underside of the seat platform, or make a simple bracket on the front face of the box and screw the separator to that. The bracket method lets you drop the separator a little lower if you need the clearance.

Always drill pilot holes in the sides of the separator before driving the screws — ABS is tough, and pilot holes stop it splitting.

Getting the height right. How high you set the separator matters. Too high and a seated man's anatomy can touch the plastic bowl; too low and you waste vertical space (a problem when you're trying to keep a van or tiny-house loo compact). We make the separator as shallow as possible to suit all builds, but as a rule of thumb — with a standard 5 cm toilet seat on a 2 cm ply top — the separator usually sits best 0–2 cm below the underside of the toilet top. It's one of those things worth dry-fitting and adjusting before you fix anything.

Cutting to fit. To bring the urine pipe as close to the front of the toilet as possible (which keeps the solids drop-zone central and comfortable), you can trim the front curve off the separator. The ABS cuts easily with an angle grinder and a metal-cutting disc, or simply a handsaw or jigsaw.

The build: a simple bucket toilet, step by step

Here's the most common DIY compost toilet design — a bucket-based box with a seat. You can build this in a day with basic tools.

Materials you'll need

• 1 sheet of 12 mm plywood
• 50 mm × 50 mm batten (about 4 m)
• A 25-litre bucket
• A 10–20 litre container or jerry can for the collected urine
• A toilet seat
• 2 hinges
• Wood screws and basic plumbing fittings (1¼" pipe and an elbow)
• A We-Pee urine separator

Tools

• Jigsaw or circular saw
• Drill
• Hole saw (to cut the toilet seat hole)
• Screwdriver
• Tape measure and pencil

Step 1: Lay out your components first

Before you cut any wood, set out your separator, bucket and urine container on the floor. These three parts determine every other dimension — the box only needs to be big enough to house them comfortably with the seat at a sensible height.

Step 2: Build the box

Cut your plywood into a five-sided box (open at the top): typically around 500 mm wide, 500 mm deep and 400 mm tall, depending on your bucket height. Reinforce the inside corners with the batten.

Step 3: Cut the seat hole

Cut a hole in the top to match your toilet seat — usually around 240 mm front-to-back, 220 mm wide — positioned so the bucket sits directly underneath. Mount the toilet seat over the hole.

Step 4: Fit the urine separator

Fix the separator under the top panel so its bowl fills the front third of the seat hole, at the right height (see above). Pilot-drill, then screw.

Step 5: Connect the urine outlet

Connect 1¼" pipe to the outlet, running down through the box and out to your container, with an elbow at the bottom to direct the flow. Keep the run sloping downhill into a jerry can or sealed container with a vented lid (a single drilled hole is enough).

Step 6: Position the bucket

Slide the 25-litre bucket inside the box directly under the seat hole, easy to lift out for emptying.

Step 7: Add ventilation

Drill a 50–75 mm hole in the back and run a vent pipe up and out. For vans or tiny houses, a stainless mushroom vent on the roof keeps water out.

Step 8: Cover-material container & finish

Keep a small bin of sawdust, wood shavings or leaf mould next to the toilet with a scoop — a scoop goes in after each solids use. Then sand, oil or varnish the box. A coat of clear oil-based varnish protects against spills and looks neat.

Bigger builds: wheelie-bin & twin-chamber designs

When you need more capacity than a bucket — a glamping site, a busy family cabin, a community garden — there are two designs we keep coming back to.

Wheelie-bin toilets

Ask a team of designers to invent the perfect composting-toilet receptacle and they'd probably land on a wheelie bin: it has wheels for manoeuvrability, it's rat- and rodent-proof, the lid makes it waterproof, and it's cheap and available everywhere. Start your design with the bin and build the toilet around it.

A wheelie bin is tall (about 1 m), but there are three neat tricks to keep the overall building height sensible:

• Nestle the bin inside the box you sit on, rather than below it.
• Add an internal step — this doesn't lower the building, but it lowers the step-up height inside.
• Dig down — sink a counter-ramp into the ground so the bin sits partly below floor level.

Twin-chamber toilets

A twin-chamber system is a large box (often recycled plastic) divided into two compartments: one in use, one resting and composting. A separator pipes the urine out to a container or soakaway. Two hatches at the back let you empty each side, and a vent pipe keeps it dry. Each chamber holds more than a wheelie bin, so you empty far less often — and by the time a full chamber is ready, the contents have become odourless compost that's not unpleasant to handle.

Complete or Compact — which separator?

Both do exactly the same job; the difference is the shape and the receptacle they suit best.

• The Complete fills the whole seat oval and looks more like a traditional toilet, which many people prefer. Its solids chute is a little narrower, so it pairs beautifully with a bucket where you want everything funnelled to one spot.
• The Compact original has a wide-open solids drop, which suits larger open receptacles like a wheelie bin or twin chamber where you don't want anything catching the sides.

As a rule of thumb: larger open receptacle → Compact; smaller bucket → Complete. Both share the same funnel and outlet.

What to do with the urine

Don't waste it. Urine diluted about 10:1 with water is one of the best free nitrogen fertilisers you can put on a garden — particularly good for leafy greens, brassicas, sweetcorn and fruit trees. Apply it to the soil rather than the leaves, and never use it neat (concentrated urine will scorch plants).

No garden? The simplest disposal options are a soakaway pit (a gravel-filled hole that lets urine soak into the surrounding soil) or, if your setup is plumbed, the foul drain.

What to do with the solids

The solids — mixed with sawdust or shavings — go to a dedicated compost heap, separate from your kitchen compost. Most experts recommend a two-year composting period before using the finished compost, and even then most people use it only on non-edible plants (trees, shrubs, flowers) rather than food crops, just to be safe.

A simple two-bay system works well: fill one bay over time, then leave it to compost while the second fills. After two years the first bay is ready to empty.

Common mistakes

• Skipping the urine separator. The single biggest cause of smelly compost toilets. Don't.
• Forgetting the cover material. Sawdust isn't optional — it's what makes the system work.
• Undersized vent pipe, or no vent at all. Air movement is what keeps the chamber dry.
• An uphill outlet pipe. The urine pipe must slope downhill to the container, or it will back up.
• A narrow outlet. Small outlets block easily — look for a generous 1¼" (32 mm) fit like ours.
• Setting the separator too high. Dry-fit and check the height before fixing (see above).
• Putting the composted solids on the veg patch. Most people don't, and most regulators advise against it. Use it on ornamentals.

Legal and regulatory considerations

In the UK, composting toilets are generally permitted for personal use without planning permission, provided they don't discharge effluent. Commercial installations (glamping sites, eco-tourism) may need to comply with local environmental-health and waste-handling rules — check with your local authority before opening to paying guests.

In the US, regulations vary by state — some have explicit composting-toilet provisions, others require a permit, and a few prohibit them. The NSF/ANSI 41 standard covers compost-toilet certification. Wherever you are, check local regulations before installing a compost toilet for any public, rental or shared-occupancy use.

See it in action

For real-world examples of We-Pee separators in DIY compost toilet builds around the world, take a look at our Customer Installs gallery — featuring vanlife conversions in the USA, glamping sites in the UK, tiny houses in New Zealand, off-grid cabins in Hawaii and aid projects further afield.

Ready to build?

The urine separator is the make-or-break component of any compost toilet. We've spent over 15 years making ours work for both male and female users, for adults and children, in vans, boats, homes, sheds and development projects in 30+ countries — each one handmade in Wales from solid 4 mm ABS and backed by a 10-year guarantee.

Two separators to choose from:

• We-Pee Urine Separator — our compact original, in white or black
• Complete Urine Separator — our all-in-one design with built-in metal filter

Both designed and handmade in the UK, and shipped worldwide — across the EU with VAT included and no surprise import fees. Got a question? Drop us a note via the contact page — we're a small team and we read every message.