Is there a limit as to how low the hands are allowed to be placed below the navel when offering Salah?
بسم الله الرحمن الرحيم
According to the Hanafi Jurists, the hands are places below the navel [i.e.in the Salah of a man].
Wa’il bin Hujr r.a. [a companion of the Prophet ﷺ] reported that he saw Prophet ﷺ placing his right hand upon his left hand below his navel in Salah. [Ibn Abi Shaiba]
Ali bin Abi Talib r.a. narrated that, it is from the Sunnah [of the Prophet ﷺ] to place the right hand upon the left below the navel. [Abu Dawood, Ahmad, Darqutni, Bayhaqi Tahawi-Ahkaam al Quran]
Imam Tirmidhi, after narrating the Hadith of placing the right hand upon the left, he states that some [people of knowledge] are of the view that the hands are placed above the navel and some are of the view that it should be placed below the navel and all [methods] are correct.
Ibn Al-Mundhir al Naysābūrī r.a. [319 هـ]narrates in his al–Awsat fi al–Sunan wa–al–ijma` wa-al-ikhtilaf that Abu Hurairah r.a. said; it is from the Sunnah [of the Prophet ﷺ] to place the right hand upon the left hand below the navel. The after he quotes Ishaq bin Rahawaih [Imam Bukhari’s teacher]; placing the hands below is the strongest view held in accordance to Hadith and closest [posture] to humbleness.
Ibn Qayim r.a. mentions in his Badai’ al Fawaid, that according to Imam Ahmad r.a, it is correct to place the hands on the navel, above the navel and below the navel.
The hands should be placed just below the navel and not too low to the extent that it is on the private pate or pubic region. If a person with an oversized abdomen finds difficulty in placing his hands just below his navel, then he may place it on or above the navel.
And Allah Ta’āla Knows Best
Mufti Kaleem Muhammad
Darul Iftaa, Jaamia Madinatul Uloom (Trinidad)
 الأوسط في السنن والإجماع والاختلاف (3/ 94)
وَقَالَ إِسْحَاقُ: تَحْتَ السُّرَّةِ أَقْوَى فِي الْحَدِيثِ، وَأَقْرَبُ إِلَى التَّوَاضُعِ.
 ص 91 ج 3
This answer was collected from Fatwa-tt.com, which is operated by the Darul Iftaa of Jaamia Madinatul Uloom (Trinidad and Tobago) under the advice and guidance of Mufti Ebrahim Desai (Daamat Barakaatuhum) of South Africa.