Marhaba !

If tenants decide to decorate a room before informing the landlord , can the tenants deposit be made non-returnable by the landlord?

It would depend on the terms of the tenancy agreement. There's usually a clause about decorating. Anyway, if the colourscheme is not too garish , you should be grateful as it'll save you time and money!

An interesting question here. As I see it, technically there is no problem in managing actions like those you, mr. Sheikh Spiiah, described.
I think it all depends on your landlord. If he is fine with helping tenants to "save money", he might agree. But in case he was planning to use these money for BTL mortgage repayment or maintenance costs, your request would be denied.

Kind regards.

Oh, hello everyone.
Here's an add to the first question: what if it was agreed verbally with the landlord that tenants' deposit is non-returnable, and they have decorated a room, but afterwards the lanlord tells it's not going to work this way, not in his plans; what to do then?
No one will return your money...

I've been thinking on this subject lately. And look, what I have found on the web.
If the landlord allows full furnishing, 10% of his large rent would be subtracted anterior to computing your tax liability.
This tax break should not be neglected.

All deposits are now protected and it is impossible to keep it for no reason. Landlord can't agree with tenant that deposit will be non refundable.

This is unbelievable! I was away for some time and there are so many replies and new developments in this forum. My thanks to you all! I do not really mind tenants redecorating, but, of course, would prefer to be kept informed and need to know where I stand in case of any disputes.