Water Saving Toilet-Mounted Basin





Introduction: Water Saving Toilet-Mounted Basin

About: We are a designer maker agency based in Cape Town. We fuse traditional industrial design with electronics and software engineering. We also try to make fun instructables in our spare time!

Have you ever thought how wasteful it is to flush the loo with drinking water, when there's so much grey water being produced by our daily activities?

This easy project adds a basin directly to the top of your toilet's cistern, so that when you rinse your hands, brush your teeth, etc, the grey water flows straight into your cistern, making it easy to flush with grey water.

Only a few tools are needed, and the system is not destructive to any part of the toilet - you can switch back at any time.

Step 1: Things You'll Need


  • Drill with hole-saw attachment
  • Jigsaw
  • Impact driver or screw driver
  • 20mm spade-bit
  • Mitre saw or hand saw
  • Pencil
  • Plumbers tape
  • Shifting spanner
  • (Optional) Belt sander
  • (Optional) Drill-press
  • (Possibly) hack saw or angle grinder


  • A sheet of opaque perspex, larger than the top of your cistern
  • A garden tap
  • Hose clamps (2)
  • Braided house connecter to fit the water outlet near your cistern (long enough to reach the tap. Ours is about 80cm)
  • Basin waste attachment
  • Plastic container or bowl, to fit the top of your cistern
  • Wooden square pole
  • 20mm dowel
  • Long wood screw
  • Large diameter metal washer

The material cost came in about $25

Step 2: Cut New Cistern Lid

  1. Place the cistern lid on the perspex sheet
  2. Trace around the lid with a pencil
  3. Cut along the line with the jigsaw. Cut slowly and carefully.

Remember to use eye protection and a mouth mask when cutting!

Step 3: Drill a Hole Through the Basin and Lid

  1. Place the plastic container on top of the lid you've cut. You may want to use a measuring tape to make sure they are centered.
  2. With a hole-saw attachment that matched the diameter of the waste-outlet, drill a hole through both the container and the lid. I drilled the pilot hole through both whilst they where on top of each other, then the larger hole separately. Make sure the drill bit and hole saw attachment don't damage the surface you're working on (consider drilling on a scrap piece of wood).

Note: it's worth looking how your flush mechanism works, and placing the waste outlet accordingly. Ours ended up getting in the way of the flushing mechanism, so we had to angle-grind it down a little.

Step 4: Attach Waste Outlet

  1. Push the waste outlet through the holes. A rubber washer came with the outlet, and we placed that on the very top of the thread, so that it sits in the basin.
  2. Tighten the screw well, so as to avoid leaks.

Step 5: Make the Tap Stalk

  1. Cut down the wooden square pole to the height you'd like you tap to stand.
  2. Using the spade-bit in a drill-press (or drill), drill a hole near the top of the wood.
  3. I also chose to sand the edges of the wood on the belt sander
  4. Cut the dowel to the length you'd like your tap to stand over the basin.
  5. Attach the tap to the dowel with the hose-clamps

  6. Insert the dowel into the hole you've drilled. Drive a woodscrew through the side to hold it in place.

  7. Apply plumber's tape to the thread, and firmly attach the braided hose to the tap

Step 6: Attach Tap Stalk to the Lid

  1. Drill a hole in the lid where you'd like to mount the tap stalk
  2. With a washer in place, drive the long wood screw through the hole, and into the wooden stalk.

Step 7: Putting Everything Together

  1. Turn off the water outlet near the toilet
  2. Disconnect the existing braided hose from the cistern
  3. Using a bit of plumber's tape around the threads, attach the braided house to the water outlet. Use the shifting spanner to make sure it's tightly connected.
  4. Slowly turn the water outlet back on, making sure there are no leaks.
  5. You're done! Enjoy saving water!

Step 8: Improvements After Testing

  • Some water creeped under the basin (not much at all). Applying silicone sealant near the waste outlet would solve this.
  • The thread at the bottom of the waste outlet was quite long, and it got in the way of the waste mechanism. Placing the waste outlet to the side would've avoided this.
  • If you have extra budget, you could use a shiny faucet that arches over into the basin instead of a garden tap on some wood

Thanks for following this instructable! I you have any suggestions, questions, or comments, please leave them in the comment section.



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    We have a be nice policy.
    Please be positive and constructive.

    2 Tips

    If there could be a way of adding some sort of float indicator that lets someone see from the outside how much water is inside the cistern, that would be quite useful in knowing how much water is needed, and whether or not its time to add extra water, or time to...uh...
    ...take the dog for a walk?
    ...empty the tank?
    ....take a zizz?
    ....drop the kids off at the pool?
    ...sink a battleship?
    ...build a log cabin?
    ...go where the king goes alone?

    If you use two or three screws the wooden stalk will not twist. Carl.

    4 Questions

    Excellent idea! How do you get the gray water into flow into the container?


    How much more complex would it be, to take an existing sink from the bathroom, and mount it on top of the toilet? It would increase the available floor space in the bathroom quite a bit.

    With a little bit of handiwork, I think it could be a good weekend project!

    1 more answer


    Since the sink would already have the drain, I guess one only need make the hole in the toilet lid. Is there a reason why you didn't drill through the original toilet lid? Is it difficult to drill though ceramic/porcelain?

    The cistern normally fills till the float rises and toggles a switch to shut off the water. How do you make sure you have enough water to flush?

    Hello! see my reply to the question below :)


    So the idea is that the toilet tank doesn't fill on its own right? Since any water you add to a full tank would just drain away, and you disconnected the fill line. So you need to manually make sure there is enough water in the tank for it to flush, right? Perhaps some form of fill gauge would be helpful? I can imagine a few options, but not sure how useful they would be. How have you managed making sure it has enough water?

    Correct - the toilet doesn't fill on its own. After a while of using it, I started to get a sense of when I had produced enough grey-water to flush. You're right, though - a gauge would be useful!


    Fantastic idea, easy to do. Using a plastic bowl allows one to see clearly how it's done, then the smarty pantsies can use what ever they want if they think its ugly. Am so trying this, and posting it on Facebook if that's OK.

    1 reply

    Of course! Thanks for sharing

    If the bathroom sink was near the toilet, one could connect the sink drain to the toilet tank to get this effect. I'm not sure that my bathroom sink uses enough water to make it worthwhile. The savings of water comes from reusing the greywater from the sink.

    Great DIY solution! They sell these in the U.S. on the Home Depot website, or Amazon, for anywhere from $99 to $150, and entire new toilets with a built-in sink on top for over $350. This is a great way to put the conservation into practice right away, and in an economical fashion. Congrats!

    I must be missing something here... you disconnected the braided hose from the water inlet, then connected your faucet to the same water inlet. All you did was turn the loo from an automatic refill to a manual refill. Where does the "grey" water come in? And why would you want to use dirty water to flush your toilet?

    I think it would be better to use the " If it's brown flush it down, if it's yellow let it mellow" method.

    2 replies

    You don't get it - the toilet filling water is coming from hand washing and teeth brushing. I can't really understand why people are saying "this won't save any water..."

    Of course it will save water - where would the water you normally use in a basin go to? It's wasted. This connects this grey water to the toilet cistern.

    My apologies, I guess I didn't express myself correctly. I did not and am not saying this will not work. I applaud you for your idea to save water and the planet. My misunderstanding comes from where the "grey" water is coming from. Your instructable says to:

    Turn off the water outlet near the toilet
    Disconnect the existing braided hose from the cistern
    Using a bit of plumber's tape around the threads, attach the braided house to the water outlet. Use the shifting spanner to make sure it's tightly connected.

    The way I understand your instructions is that the drinking water that was going into the toilet tank is now going into the garden tap you added.

    Where does the "grey" water come from? Do you carry it from a basin in the sink where you washed your hands or brushed your teeth? Do you have a separate water collection for "grey" water vs. "black" water?

    I've enjoyed seeing your project.

    Given the way a standard toilet is designed to flush and re-fill. this solution would not save/re-use any water - though it would add grey water to the water used to flush your toilet.

    If you look into the toilet tank, you will find float mechanism that controls the fill valve for the toilet so that the incoming water shuts off once the tank has been filled to the proper level.

    If you were to over ride this float (hold it down, for instance), the water would continue to flow into the tank. At some point the water level would rise above the elevated drain tube designed to prevent over-filling should the float valve fail.

    Thus, after you flush your toilet, the tank is automatically re-filled. If you were to use this basin, the water running down its drain would fill the tank a bit more than the level set, then run down the overflow tube into the toilet and out the toilet drain.

    Another short-coming is the implicit requirement that one use this toilet-mounted sink to wash their hands and brush their teeth. While, I suppose, not necessarily unsanitary - definitely less than appealing.

    Although, as my wife reminds me, they use a similar combination fixture (usually out of stainless steel) in many prisons.

    2 replies

    Hello! The inlet to the cistern has been disconnected here, so the toilet does not automatically fill. The only way it fills is through this basin. Although, I have been thinking that if we had left it connected, if you washed your hands quickly after a flush, it would add to the cistern while it is still filling, saving a little water (although I haven't tried this). Interesting note about prison fixtures, I'll have to take a look at those!

    My apologies, then, for not reading fully!

    Better still, just use a bucket of rainwater.
    Main advantage is that it's soft water, so no descaling chemicals if you live in a hard water area.
    I have sized it up a bit to cover a lot of other things, but so far, used about 40-tonnes (40000-litres) of rainwater. Bills cut to absolute minimum. Makes all sorts of washing a pleasure.
    Have to agree with comment that it makes no sense to flush waste away with drinking water - takes a he amount of effort to produce, then rendered useless in the WC.

    2 replies

    Perhaps if a second cistern, filled with rain water or such were added, then connected to this faucet, you have a pretty awesome system. It should be fine for teeth brushing and hand washing, unless you live in Beijing or somewhere with air pollution to worry about...

    For second cistern, read second large header tank in the loft area.
    It's all part of a much larger rainwater collection system which has evolved over the years and includes a sand filter at the roof downpipe end.
    To keep the water sterile, I add a dash of sodium hypochlorite (domestic bleach).
    I'm not sure that I would really want to use it for teeth brushing, but for everything else, it's fine. A lot depends on roof construction and concrete tiles in particular collect a lot of rubbish. In well over ten years use, I have never had a health issue that could be related. As an ex-water scientist, I have no issues with the water as used. My potable water consumption has shrunk from the national average of 150-litres per day to below 10-litres. It's a win-win system and has much more significant benefits to the whole water supply and waste disposal industry that it is too long to list here. What it does do is turn rainy days into raw material collection days

    why not use something more sustainable (and nicer to the eye) than a plastic box, for the basin? a metal salad bowl, for example, ok, a bit more tough to cut but not impossible either. and if it's about being cheap i guess you can find one at a Good Will Store, or flea market.
    good idea, nonetheless.

    5 replies

    omg this is awesome!! :D


    I'm sure that a lot could be done for aesthetics. Nonetheless, this project really demonstrates a great concept - the sooner a bathroom gets fitted like this, the sooner water will start getting saved.

    Speaking of sustainable - correct me if I am wrong, but plastic requires much less energy to produce than steel. Ceramic would be the most sustainable. ^^

    I was hoping the little pot plant on the side could be included somehow...

    not sure plastic requires less energy to produce, but for sure, those box can break easily after several years, wich would not happen with a stainless steel bowl (or a ceramic salad bowl). you can already find in store those toilet tank with a small sink on the lid, but if your goal was to demonstrate it was possible to do it at home with less money, yeah, it's a success. :)

    Great idea! The bowl shape would probably be also easier to wipe down.