Khloe Kardashian, Kris Jenner's Side-by-Side Mansion Construction Underway

Khloe Kardashian and Kris Jenner are about to be just a stone's throw away from each other -- their next-door mansions are in the midst of construction ... and they are HUGE.

Khloe and her momager are building side-by-side modern farmhouse estates in an exclusive enclave in the Calabasas area. These new aerial shots show there's still a ton of construction going on, and it looks like daughter built a little bigger than Mom.

When it's all complete ... both Khloe and Kris will have enormous swimming pools where they can entertain, and do plenty of IG photo shoots.

According to reports ... the massive lot used to be one single estate. After it was sold in 2012, another local developer razed the house ... but it sat vacant for years. The new developer's now putting the finishing touches on Khloe and Kris' houses.

We don't have exact numbers on how much Khloe and Kris paid -- but there are reports they each plunked down north of $10 million.

We're told Khloe's home -- on the left -- is 18,000 square feet and features 8 bedrooms and 9.5 bathrooms. There's also an 8-car garage, covered patios, guesthouse, movie theater and gym. Kris' place is a massive 17,000 square feet.