three things.
firstly has any other animal been in your house like a mates dog or another cat? do you own this property and is it possible they had cats?

you may need to clean the areas really well with a special enzymatic cleaner like this (Nature's Miracle Stain and Odour Remover Gallon) or with a mix of bio washing powder with a steam carpet cleaner and then cover the area with vodka/gin and leave to dry.

secondly, it's possible she doesn't like the litter you use or the place the tray is in. They need to be in a quiet low traffic area, maybe put some in these corners where she's been going once you clean them. Also maybe have a couple round the house and use different litter and see which one she prefers. there are all types out to try. If you use a covered litter tray she may not like the cover, or if you use a non covered she might want a cover, but if you change to a covered tray be sure to take the door off at first and then add the door after a few weeks of her using it.

thirdly, the other possiblity is that she has a urinary tract infection or swelling in the urethra. She will need to be seen and cared for by a vet for this. Personally if this has been going on for long i would take her into the vets for a check over and a course of antibiotics to see if this resolved the problem.

She may also be having a bit of a stressful time, for reasons you may not understand. new guest in house, children, noise or pet outside, storms, etc... Maybe investing in a Feliway Diffuser for her to calm her a bit. you can read more about Feliway here: http://www.feliway.uk.com/
I use it for my girls and they love it.

If you find it's a urinary problem many things could be the culprit that you could avoid yourself. some are just caused by anatomy, but most cases are caused by stress or diet. Its possible the food you are using isn't the best for kitty and you might want to change her diet.
If you choose to do this speak to your vets about a urinary diet to try for a little while to get her back to normal and then think about trying a healthier diet from the shops or online. There are many choices and lots of previously started topics on this forum that you can search for or if you prefer just start another topic in request for ideas. we're all happy to help.


hope this helps and don't hesitate to ring your vets for advice they are always happy to answer questions for you if you are worried and out of ideas. all the best! let us know how you get on.