As the Islamic dominions extended progressively, Umar ordered that mosques should be built in all conquered territories.
In the newly founded cities of Kufa and Basra, Jami Masjids were built in the centre of the city and smaller mosques were built in each tribal quarter.
In the case of smaller towns in Iraq and Syria, a mosque was required to be constructed in each town. According to one account as many as 4000 mosques were constructed during the caliphate of Umar.
Umar had the sacred mosque at Kaaba extended. In 739 AD Umar purchased the surrounding houses at state expense. These were demolished, and the area under them was included in the mosque. Heretofore there was no wall round the mosque. Umar had a wall constructed for the first time. Heretoforc the mosques were not lit. Umar provided lights for the mosques for the first time.
Formerly the cover of the Kaaba was of ordinary cloth. Umar had the cover made of a superior and finer cloth manufactured in Egypt.
The bounds of the Haram, the sanctuary of the Kaaba extended to three miles in one direction, and seven to nine miles in other directions. The boundaries were not defined, and there was the risk of this area being encroached upon. Umar had the area surveyed, and the boundaries were demarcated. Stone pillars called Ansab were fixed to mark the boundaries.
Umar extended the Prophet's Mosque at Madirnq as well. In 739 AD, the same year as the Kaaba was extended, Umar purchased the houses that surrounded the Masjid i-Nabvi. After demolishing them, the area was utilised for the extension of the mosque.
Abbas whose house also surrounded the mosque refused to sell his house. He sued the state in the Court of the Qazi Ubayy b. Kab. The Court gave its verdict against the state, and held that the property could not be acquired compulsorily. Umar accepted the verdict of the Court. Thereupon Abbas voluntarily gifted his house for the extension of the mosque. Umar accepted the gift gratefully, and provided alternative accommodation to Abbas.
As a result of extension the length of the mosque rose from 100 to 140 yards while its width rose from 60 to 80 yards.
Umar was the first to provide lights for Masjid-i-Nabvi. Umar also made arrangements for the burning of the incense in the mosque. The floor of the mosque was paved and covered with mats.