If other members of a guild other than the guild leader have a guild house I think that should add additional spaces to the guild.

If you compare gem prices then it would be worth 5 places (495 v 1250).

If you compare the gc cost of certificates with the cost of building one then it would be worth 8 places (400 v 3000 + 5k marble + 5k hwp).

Or you could just go the whole hog and add 15 places