Peugeot 206 leak in rear footwell water builds up between door and rubber seal

lots of ways, to leak rain (or other things)
windshield glass cracked
door seals bad or door warped
heater core, leaking engine coolant, (not water AF50% coolant)
the water in hand feels slimy, is not rain.!!
the cars most have cowl vent area in front of windshield glass.
this cowl when opened UP we see drains if the 2 or more drains Are clogged up with leaves or nests. the cowl floods like a bath tub
and overflows the HVAC fresh air vent, port, (a coffer dam it is)
and flows to the hVAC blower and to the floor., bidda bodda boom.
or super bad rusted car bodies sure do leak.
never ask the blind we can not see your car, blind
find shop tech to find it , living hands on work.

Once you have checked it isn't a car window that is slightly open there is three main reason for water in the foot wells. Sun roof, door seals and heater matrix.
If you have a sun roof fitted, check it is closed correctly. Open it all the way and check the running grooves it sits in for dirt and blockages. There are drain holes in the runners so try cleaning the runner grooves with an old toothbrush.
Open the front door on that side of the vehicle. Take a good look at the rubber seal the door presses against when it is closed. Any damage or splits can create leaks into the vehicle. Check the rear door seal for the same thing. If you can, check all the other door seals and the trunk/boot seal as well. These seals are usually a tight fitting 'push on' type which you can replace yourself.
This will sound silly but does the 'water' smell of anything? The heater matrix is under dashboard and is what supplies the heat to the heater blower controls on the dashboard. If this is leaking the leak will have a sweet smell as this is coolant from the engine. This is a big repair as you often have to remove the whole dash to repair it.

it is best not to waste time and turn to professionals in this field

If it is inside the car in one footwell it will be one of three things. It will be either a leaking pipe to the rear windscreen washer jet, a worn door seal rubber or the heater matrix behind the dashboard.
Get into that footwell and using the back of your hand start at the floor and feel the carpet for wetness. Work your way up the carpet towards the underside of the dashboard. Your looking for the point where the carpet goes dry. Try this near the centre of the car and nearest the door. Floor only suggests washer pipe. Wet around the door is possibly a door rubber and high under the dash is most likely the heater matrix. This may seem an odd question but does the wetness have a smell? A rotten egg smell suggests screenwash. A sweet smell could be coolant.

HI yes 100% brake servo plate its a common on corsa c and combo vans water in the driver footwell and rear driver footwell. dealers are charging a fortune to replace the plate and you will see lots of videos on youtube on how to repair it without taking out the brake servo. fact is the servo needs to come out to carry out a good repair.

OK ideas & to check:

is there water getting in through the rear doors as the inner door membrane is not watertight? if this is the case will need sealing.

Sunroof: if there is a sunroof check ducts are not blocked.

Tailgate lights: Is there water sloshing around inside any of the lights, as this may cause the water to breach their seal & run from boot under seat to footwell....

Is it just rear footwells or are the front wet too? if so check again ducts from sunroof as the hoses come out of the wheel arch. OR have you had new windscreen - poor fitting windscreen (even with the best intentions) causes it to leak. Or heater matrix (this will cause front wells to be wet & will run backwards: check your water levels in expansion tank, if it looses water could be faulty matrix. IF leak is rain water it tastes different to the water / antifreeze in tank...

does this happen only after it rains?
check under the wheel well or near by for rust the other place maybe around the rear window seals leaking in under them.

i have been fixing peugeots for 12 years and never heard anything like that.
it is mrer then likley a faulty crank position sensor, coil pack or quite simply the fuel cut off switch needs resetting which should be located under the bonnet on the n/s (black unit with a red rubber top) just press the top and try starting the car..

if this helps let me know
cheers
big mike

It's probably coming from the sunroof drains being clogged.

Open the front door, look closely, you will see a rubber tube sticking out 1 inch. Cut off the tips so the tube is wide open.

May be you need to replace the door and boot rubber seals. Check any gaps between affected area surfaces and if possible carry out the necessary work to stop the leakage. If the problem is still there then please describe in detail when and how i.e in rain from top or bottom side etc etc.