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 know to build a compost loo that actually works — from the materials list to the urine separator, the chamber, the ventilation, and where the waste goes.

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 separate — 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. We'll cover each.

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 urine separator that catches reliably for both male and female users, in all sitting positions, with a flat outlet bottom, smooth easy-clean surface and a properly sized outlet hole, is harder than it looks. We've watched a lot of DIY versions get scrapped after a month of awkward use.

Either way, when choosing or making a separator, 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 hole — ours is 30mm diameter to handle standard 1¼" push-fit waste pipe without blocking
• Dimensions that fit under a standard toilet seat hole — roughly 170–330mm wide
• Strong, durable plastic that won't crack or stain over years of use

Our £31 Urine Separator is our lightweight original (white or black), and the £40 Complete Urine Separator is the newer all-in-one design with a metal filter. The FAQ page covers which to choose.

2. The solids chamber

The solids chamber is whatever the poo drops into. For the simplest setups, a 25-litre plastic bucket works perfectly — they're 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.
• Twin-chamber wheelie bin setups — larger capacity, alternate between two chambers so one composts while the other fills.
• Built-in concrete or block chambers — for permanent installations like glamping toilets or eco-resort facilities.

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 any residual moisture, covers the deposit, and starts the composting process. 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 50–75mm 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 prevents any residual smells from escaping into the room.

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

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 12mm plywood
• 50mm × 50mm batten (about 4m)
• A 25-litre bucket
• A 10–20 litre water container (for collected urine) or jerry can
• A toilet seat
• 2 hinges
• Wood screws and basic plumbing fittings (1¼" push-fit pipe, 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: Build the box

Cut your plywood to make a five-sided box (open at the top): typically around 500mm wide, 500mm deep, and 400mm tall. The exact dimensions depend on your bucket height and how tall the seat needs to sit. Reinforce inside corners with the batten.

Step 2: Cut the seat hole

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

Step 3: Position the urine separator

Screw the urine separator to the underside of the top panel, positioned so its front collection bowl fills roughly the front third of the toilet seat hole. This is the position where urine reliably enters the separator for both male and female users when seated.

Step 4: Connect the urine outlet

Connect a length of 1¼" push-fit waste pipe to the separator's outlet, running down through the bottom of the box and out to your urine container. Use an elbow at the bottom to direct the flow into a jerry can or sealed container with a vented lid (a simple drilled hole works).

Step 5: Position the bucket

Slide a 25-litre bucket inside the box directly under the seat hole, positioned to catch the solids that drop through the separator. The bucket should be easy to lift out for emptying.

Step 6: Add ventilation

Drill a 50–75mm hole in the back of the box and run a vent pipe up and out through the wall or roof. For van or tiny-house installs, a stainless mushroom vent on the roof keeps water out.

Step 7: Cover material container

Keep a small bin of sawdust, wood shavings or leaf mould next to the toilet with a scoop. Users add a scoop after each solids use.

Step 8: Finish

Sand, paint, oil or varnish the outside of the box as you like. A coat of clear oil-based varnish protects against any spills and looks neat.

What to do with the urine

Don't waste it. Urine, diluted 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 don't use neat (concentrated urine will burn plants).

If you don't have a garden, the simplest disposal options are a soakaway pit (a gravel-filled hole in the ground that lets urine soak into the surrounding soil naturally), or down the foul drain if your setup is plumbed.

What to do with the solids

The solids — mixed with sawdust or wood 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 only use it on non-edible plants (trees, shrubs, flowers) rather than on food crops, just to be safe.

A simple two-bay compost system works well: fill one bay over time, then leave it to compost while the second bay 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.
• Putting the separator outlet at the wrong angle. The outlet pipe needs to slope downward to the urine container. Even slight uphills will lead to backups.
• Using a separator with a small outlet hole. Sub-30mm outlets block easily. Don't.
• Composting the solids and adding them to your veg garden. Most people don't, and most regulators advise against it. Use the finished compost 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 regulations and waste-handling rules — check with your local authority before opening to paying guests.

In the US, regulations vary by state. Some states have explicit composting toilet provisions, others require a permit, and a few prohibit them entirely. 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 builds from vanlife conversions in the USA, glamping sites in the UK, tiny houses in New Zealand, off-grid cabins in Hawaii and aid projects in Haiti.

Ready to build?

The urine separator is the make-or-break component of any compost toilet build. We've spent 15 years making ours work for both male and female users, for adults and children, in vans, boats, homes, sheds and developmental projects in 30+ countries.

Two separators to choose from:

• We-Pee Urine Separator — £31 — our compact original, in white or black
• Complete Urine Separator — £40 — our all-in-one design with built-in metal filter

Both designed and manufactured in the UK. Ships worldwide. Got a question? Drop us a note via the contact page — we're a small team and we read every message.