Why do some mosques have separate entrances for women?

Mostly for privacy and practical reasons, not because Islam requires it. A separate door gives women a more private way in, keeps busy entrances from getting crowded, and often leads straight to a women's hall or wudu area. Women are welcome in mosques, and layouts vary a lot from one mosque to the next.

Women have always belonged in the mosque

Women prayed in the Prophet's mosque, and he told the men not to stop them. He said: "When women ask permission for going to the mosque, do not prevent them" (Sahih Muslim 442). So a separate entrance should never be a way of keeping women out. It is only a way of organising the building.

Why some mosques add a separate entrance

Privacy: women can come in without passing through the men's shoe area or the men's wudu space.
Crowd control: at Jumu'ah and Eid the main door can be packed, so a second door reduces crowding.
Direct access: some women's entrances lead straight to a balcony, a separate hall or a women's wudu room.
Community culture: many mosques follow the customs of the communities who built them.
Building design: in converted houses, churches or warehouses, an extra door may simply be the easiest way to create two areas.

Is it based on the sunnah?

There is no Quran verse or hadith that requires a separate entrance. One report in Sunan Abi Dawud records the suggestion of leaving one door of the Prophet's mosque for the women, though narrators differ over whether it was the Prophet ﷺ or 'Umar who said it (Sunan Abi Dawud 462). Scholars generally treat it as good organisation rather than a rule.

The Prophet ﷺ also described the order of the rows in congregation, with men's rows first and women's rows behind them (Sahih Muslim 440). That is why many mosques place the women's section at the back or in a separate room. See Where do women stand in the prayer rows?

How layouts vary in the West

One shared entrance with a partition or curtain inside the hall.
Separate women's entrance leading to a hall, balcony or ground-floor room.
Same hall with the women's rows behind the men's.
Mixed arrangements that change on busy days such as Fridays or Ramadan nights.

If you are a woman visiting a mosque for the first time, look for signs near the door, or ask a volunteer or a sister outside. See Can women pray in the main hall of the mosque?

If the arrangement isn't working

A women's entrance should not mean a dark, cramped or poorly heard space. If yours is, it's fair to raise it politely with the committee. Many are open to feedback and simply haven't heard it from the women who use the space. Mosque design is explained in a general article. See What are the parts of a mosque?

Key points

Separate entrances are about privacy, crowding and building design.
Women are welcome in mosques and must not be prevented (Sahih Muslim 442).
No text requires a separate door, so layouts differ widely.
If the women's space is poor, ask the committee politely to improve it.

Tip: If you can't tell which door to use, ask at the front desk or wait for another worshipper. Nobody minds a newcomer asking.