If the land of a lavatory has not been endowed to be used as toilet, it is permissible to annex it to the mosque to be used by people who come to offer their prayers in the mosque. However, if it is used endowed to be used as toilet, that place cannot be changed into mosque but there is no objection to praying therein. The following question and answer which late Grand Ayatollah Gulpaigani has answered are good to read: "There is a mosque which has been expanded but the mosque used to have toilets which have now been destroyed and its land has been cleaned. Some people who come to the mosque to offer prayers exercise precaution about performing their prayers on that piece of land where there used to be a toilet. You are kindly requested to let us know what we should do about praying there." Answer: "Given the supposition made in the question, the said land is meant to be used for a special purpose; it is, therefore, not permissible to annex it to the mosque to use for a different purpose. Thus, if it is made part of the mosque, it will not be treated as mosque but there is no problem in offering prayers there."[1]
Of course, if only the underground is meant for lavatory and bathroom, it is permissible to build a mosque on its upper floor or annex the upper floor to the mosque.[2]
As well, the place which has been endowed (given as waqf) to be used as Hussainiyah cannot be annexed to the mosque nor can it be treated as mosque. Supreme Leader, Grand Ayatollah Khamenei says in an answer to a question: "If a Hussainiyah has been endowed to be used as Hussainiyah, it is not permissible to change it into a mosque, nor is it permissible to annex it to the mosque."[3] Places which have not been endowed can be used for other purposes. Thus, a Hussainiyah or a toilet can be changed into a mosque.
When it comes to how a piece of land or a building is treated as mosque, I should say that it is possible in two ways: One is to recite a special formula of endowment. For instance, if the owner says "I have made this site or building a part of the mosque", that place will then be considered as part of the mosque." Second, an action like performing two rak'ats of prayers performed with the intention of making a place part of a mosque would be sufficient provided that the action signifies the owner's intention of making it a waqf property."[4]

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