The toilet was installed under the stairs a year ago and has hardly been used - the bowl keeps filling after flushing and overflows, I cant shut it off. It flooded the down stairs twice now - all over newly installed laminate flooring in hall.

Water is not draining completely from the toilet line to the sewage tank. If there is a 'low/little slope' section anywhere in that drain line, solids WILL settle and collect there and cause water to back-up in the toilet drain line. Then the toilet will not flush completely or at all until the 'soft blockage' is cleared out and the drain line emptied. Al 'gravity drain' toilets require a free draining/empty drain line that allows flush water to drain unrestricted. A basement toilet has a pump that 'forces' flush water up and out with enough pressure to sometimes force out the 'soft clog.'

hi Patricia.
Overview Toilets haven't changed much in the last 80 years. After a flush, water still fills a tank, lifting a float that shuts off the water when it reaches a certain level. A lever still opens a flapper to cause the flush, falling back into place when the water level drops. So it's no surprise (nor any consolation) that we face the same flush problems today that our grandparents did. Sometimes the flush is too wimpy, sometimes the toilet keeps running, and sometimes the bowl doesn't refill.
Our ace plumbing consultant has a simple four-step strategy to solve 95 percent of these problems. You can complete the first three steps in five minutes. That'll solve most problems. The fourth step is usually easy too, but not always. More on this later. These steps work for most toilets but not for pressure-assist models.
Check the fill tube

Overflow tube problem

Push the fill tube firmly onto the fill valve. Make sure the fill tube sends water into the overflow tube.Remove the tank lid and find the fill tube. It's a small flexible tube that runs from the fill valve to the overflow tube. While the tank refills, this tube squirts enough water down the overflow tube to refill the bowl after the completed flush. If this tube falls off or the water stream misses the overflow tube, the bowl won't fill and your next flush will be wimpy (that is, won't develop a strong siphon). Reattach the fill tube and make sure it perches about 1 in. above the rim of the overflow tube. Flush the toilet and watch the water stream to make sure it goes down the overflow tube.
Adjust the fill height

Check the float

Adjust the float to set the water level. Pinch the clip and slide the float up or down on the rod. Keep adjusting the float until the water shuts off at the proper level.The water level in the tank is controlled by an adjustable float. A float that's set too low produces a weak flush; if it's set too high, water spills into the overflow tube and the fill valve won't shut off. The toilet keeps running. Look for the fill level mark on the inside back of the tank and mark it on the overflow tube so you can see it more easily. If you can't find it, measure down about 1 in. on the overflow tube and make a mark. Then flush the toilet and see if the water reaches and stops at that mark. If not and the toilet keeps running, adjust the float up or down. If you have an old toilet, you'll have to bend the brass rod that connects to the float ball to make adjustments. But with newer toilets you usually turn a screw or slide a clip along a rod. Flush the toilet after each adjustment.
Also make sure that the water level is at least an inch below the C-L (critical level) marked on the fill valve. You can adjust the height of many valves to raise or lower the C-L.
Occasionally the fill valve simply won't shut off, which means that it's defective. If so, turn the water supply off at the shutoff under the tank. Buy a replacement valve (sold at hardware stores and home centers). You don't have to match the old one; many, like the one shown, fit most toilets. It's a 15-minute change-out.
Adjust the flush handle/flapper chain

Flapper chain fix

Adjust the chain to leave a little slack with the flapper closed. Then cut off the excess, leaving about an inch.A chain that's too short or tangled won't allow the flapper to close and water will continue to leak into the bowl. This causes the fill valve to cycle on and off to refill the tank. A chain that's too long, or a flush rod that hits the the tank lid, won't open the flapper wide enough to stay open for the full flush. You'll find yourself having to hold the lever to complete a good flush.
To avoid these problems, adjust the linkage in the chain to leave only a slight bit of slack when closed. Cut the chain at the rod to leave only about an inch extra to reduce the potential for tangles. Then put the tank lid back on and make sure the flush rod doesn't strike the lid when you press the lever. If it does, bend it down slightly and readjust the chain.
Replace the flapper

Change the flapper

Unsnap the old flapper and take it with you to the store to find an exact replacement. In addition to the closest replacement, pick up a "universal" type.If you've completed the first three steps and your toilet keeps running, chances are you have a worn-out flapper. Turn off the water, remove the old flapper and take it to the store to find an exact replacement. (Hardware stores often carry a wide variety.) Most flappers snap over ears on the overflow tube. Others have a ring that slips over the tube.
Now here's the catch. You may not find an exact match. The range of flapper styles has mushroomed over the last 15 years, and you may find 15 to 20 flapper options on the store shelf. Some packages include specific brand and model information (so note yours before you leave home). Others have a "universal" label. If you can't find an exact replacement, try the closest one and pick up a universal type as well. They're cheap, and the extra one just might save you a second trip to the store! (Avoid the "adjustable" types unless you're replacing an adjustable one.)
Install the new flapper and make sure it opens and closes freely. Then test it. If the toilet keeps running or runs intermittently, you're not getting a good seal. Try a different flapper if the toilet won't stop running.
If you just can't find a flapper that seals, consider replacing the entire overflow tube/flapper. On most toilets (two-piece), this means removing the tank. It's not difficult and you don't need special tools. It'll take you about an hour, and you'll avoid that expensive plumber service call.
if you go to this link it will show you step by step on how to fix it. How to Stop Running Toilet Family Handyman

Troubleshoot Your Toilet Without a Plumber

www.popularmechanics.com/home/how-to/a4117/4303948/

Overflowing, constantly running and sweating toilets can test a home owner's sanity. ... take for granted--until we can't take a shower or make the toilet flush properly. ... If none of the above does the trick, you may have a misadjusted or faulty fill ... the waterlevel a half inch below the top of the overflow tube inside the tank." ...

Emergencies - Overflowing toilets - Toiletology 101

www.toiletology.com/emergenc.shtml

You can't just lower the water level in the tank; you need to displace some of ... and the ballcock and flapper are working properly and shut off when the tank is full. ... The bowl is filled with the water that goes into the overflow pipe. ... Unfortunately, when a toilet bowl is clogged and the water level rises and doesn't flush the ...

How old? Who installed? Any warranty? I'd follow that route first.
If not warranty I'd try to get the installer to be interested. Perhaps he will have a "red face" an help out.

fill valve not shutting off water running over overflow tube set or replace fill valve flapper valve leaking replace flapper valve cracked and leaking flush valve assembly replace

Have you check the toilet for proper water level in tank. Level will be near mark on overflow tube or within 5/8" from top or it adjust float if not to get it there.

Next is the fill line 1/4" poly line connected to fill valve to overflow tube to allow proper amount of water in bowl.

Port's under the rim are clear on not clogged you can use a nail or paper clip to push through thes to clean out. You can't see them they are under the rim of the bowl.

If something fell in bowl and got flushed it could be hanging up in the trap in the bowl. If you have some rubber gloves check out drain in bowl before trying to use something to push it further in the drain.

If nothing found maybe try a 6 ft auger you can get these fairly cheap and they are handy to have around.

The small black tube that goes into the overflow is used to fill the bowl after flushing. Would check to make sure that water is coming out of that tube at full force or if its coming out at all.

Check the fill line in the tank and make sure it is connected from fill valve to overflow tube. After you flush the toilet it fill's the bowl and tank.

It should be a line about 1/4 ' in size coming from fill valve to overflow tube.

There is either a screw on the top of the fluidmaster, or the float can be screwed out further. If it has a screw on top of the fluidmaster, screw it in to make the valve shut off earlier. Keep adjusting and flushing until it shuts off before it overflows. Hope this helps...

for the upstairs if the valve is fully on there is not much you can do other than replace water service to the house or heaven forbid repipe.Valves usually don't restrict fill volume.But you can take the supply loose from the tank and stick it in a bucket turn the valve on and check the flow.this should narrow it down.Down stairs repair or replace flush valve