Prayer is an important matter, and it is not permissible to be careless about performing it. Rather it must be done regularly at the proper times, as Allaah says (interpretation of the meaning):
“Verily, As‑Salaah (the prayer) is enjoined on the believers at fixed hours”
“Guard strictly (five obligatory) As‑Salawaat (the prayers) especially the middle Salaah (i.e. the best prayer ‑ ‘Asr). And stand before Allaah with obedience [and do not speak to others during the Salaah (prayers)]”
If a person can find water and is able to use it, it is not permissible for him to do tayammum. Concerning that, the Messenger of Allaah (peace and blessings of Allaah be upon him) said: “The prayer of one of you will not be accepted, in the event of minor impurity, until he does wudoo’.” Narrated by al-Bukhaari (6954) and Muslim (225).
It is not permissible to do tayammum unless there is no water available or there is the fear of harm if it is used. Allaah says (interpretation of the meaning):
“But if you are ill or on a journey, or any of you comes after answering the call of nature, or you have been in contact with women (i.e. sexual intercourse), and you find no water, then perform Tayammum with clean earth and rub therewith your faces and hands”
Based on this, it is not permissible for you to do tayammum when you are able to use water. You can do wudoo’ inside the washroom, because there is nothing wrong with doing wudoo’ there. And the one who fears Allaah, He will grant him a way out.
And Allaah knows best.