Assalamu aleikum,

If a Muslim says or believes that Allah is in the heaven, sitting on a throne and has a form. Has that Muslim committed apostasy from Islam or just committed disbelief in theology without falling in the concept of being an apostate. Is there any difference between disbelief and apostasy in Islam?

This issue is subject to an important legal axiom:
There is a difference between judging a deed or believing it is Kufr [disbelief] and between declaring a certain person a Kafir [disbeliever).

This means that a statement may be deemed as kufr in itself though the person uttering it is not a Kafir. This may be due to reasons such as ignorance, misinterpretation, compulsion and so forth. An excuse may sometimes concern a certain individual or the public due to prevailing ignorance among people.

Likening Allah Almighty to His creation is an act of Kufr because it involves denying His words,
There is nothing like unto Him" [Qur'an 42:11], and
"Nor is there to Him any equivalent" [Qur'an 112: 4] and other verses, hadiths and evidences from the Shari'ah [Islamic law).

However, a person who unintentionally utters a statement that may be interpreted as comparing Allah to His creation, does not deny the verse even if his words do imply the comparison. In this case, we are not to deem this person as Kafir.

The ruling
A person is declared as Kafir only by denying the words of Allah and His Messenger (peace and blessings en upon him and his family). However, we are to explain to him that this is a deviation from the sound belief.