Derek Fisher and Gloria Govan Plan L.A.-Area Home Expansion -

Derek Fisher and Gloria Govan aren't just tying the knot, they're blending 2 families together ... which means adjustments need to be made to their house.

The couple has already laid out plans to make more room for their soon-to-be blended fam ... according to L.A. City docs. The 2 got a permit in January for Derek's Tarzana pad to remodel 2 bedrooms, and build an additional bathroom on the second floor.

Fisher's home is already 5,110 sq. ft with 5 beds and 5 baths, but when you take into account he's got 4 kids and Govan has 2 ... the more space (and bathrooms) the merrier.

Estimated cost -- $10k.

We broke the story ... Derek popped the question to Gloria earlier this month surrounded by their kids. Obviously, she said yes. Her ex and father of her sons, Matt Barnes, even weighed in on the engagement ... saying he was happy for everyone.