cat weeing in house - tried everything

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hello, we're at our wits' end - for the last six months our 6-year-old cat has been weeing in the hallway on the mat by the front door.
we've consistently cleaned it, removed the smell with bicarb and sprayed it, cleaned it with lavender/citrus, etc.
we've put her out straight after her meals morning and evening and we know she's using the garden to wee/shit. we know she can and does use the cat flap of her own accord so it's not that.
we have been stern with her; we have been nice to her.
but we are desperate - it's so miserable never to know if/when your house is going to reek of cat wee.
has anyone any idea why she might be doing it? and what we can do?
pleeeeeeaaaaaaaase

for some reason she is marking that spot in your hall its usually anxiety related, is there a new cat on the block who could have marked the outside of your door and she can smell it even if you cant, try washing the outside and inside of the door with citrus smelly stuff

As far as Ive always been told is citrus products should be avoided as they enhance the smell of the amonia in their urine so even though we cant smell it as humans, the cat can still. Try cleaning the area with surgical spirit.

Is there a catflap in your door? if there is access into the house, its possible your cat may be being bullied by another cat coming into your house.

Forgot to add, it would explain why shes urinating outside but also inside, sounds more like territory marking than the actual need to urinate.

Has she been checked for a UTI ? My cat wees in the house when he gets stressed which triggers a UTI in him.

Our last cat nightmare was because of another cat coming in for his food. So we started keeping our cat out of the room where the cat flap is. Gave him a litter tray. Moved his food at night so there was nothing for the other cat. And put a tray of water under the cat flap so the other cat got wet legs if he came in.

Clean the area with biological washing powder and spray with surgical spirit mixed with water.

Our cat started weeing in the house. Dh took him to the vet this afternoon and he has a uti. Long lasting antibiotic injection and £68 later and hopefully it's all sorted.

Hello everyone, I'm really grateful for all the suggestions and will now
a) clean the area with surgical spirit/biological washing powder
b) take her to the vet to check for a UTI
(Reading what you have all said, I think it may have started by being territorial but then as pancake flipper says become a UTI.)
thanks so much and fingers crossed!