Zuckerberg's plans to keep the four homes as part of a larger compound where family and friends could stay is what the board took issue with, saying it would ruin the single-family home feel of the Crescent Park neighborhood.
A single family using all four properties violates the city's efforts to protect single-family homes, according to the board.
And one large compound also violates local zoning codes with single lots not being allowed to exceed 20,000 square feet, the board said.
The four homes together exceed 60,000 square feet, not including Zuckerberg's primary residence, which would make it 'even bigger and even more out of compliance,' according to board member Wynne Furth.
The architect firm working with Zuckerberg said the project aims to expand their client's space for 'residential function,' The Mercury News reported.
The Zuckerbergs currently have friends living in some of the homes while the others are used as an extension of their living, cooking and entertaining quarters, Kathy Scott of the Walker-Warner Architects firm said.
'The idea is just to expand our client's capacity to enjoy the property - sharing time with friends and family, having more outdoor space to play,' Scott said.
'The current property is quite restricted and so this is just giving them more space for their residential functions.'
But board members say the four homes are not separate residences or 'credible' single-family homes, and instead operate as part of a larger compound.
'This is something you might find in Atherton - a large estate, a series of guest houses, recreational facilities, movie theaters surrounding a house,' board member Peter Baltay said.
At this time, there are no plans to merge the lots or amend their lot lines to maintain flexibility and independence for future homeowners, Scott said.
Each house consists of basic home essentials such as a bedroom, kitchen, laundry facilities and a bathroom to be considered as a standalone.
However, Furth said layout plans suggest otherwise with one home consisting of two bedrooms, three washers and dryers, 'a really nice vegetable garden' and a project room.
Another, described as a 'great place for a party,' would be rebuilt to include a 40-by-30-ft great room, a large room in the basement and a service kitchen.
One board member believes if the homes were put on the market individually, they would have a hard time selling, according to The Mercury News.
'Some of them are very unusual,' Robert Gooyer said. 'In the context, if you take all four of them, they make sense.'
While the board, which is an advisory committee, recommended the city reject the proposal, the city's planning director can still approve the plans, overriding their decision.
However, architects working in Palo Alto told Curbed SF that rarely happens.
Following Thursday's review board meeting, Zuckerberg is reportedly going to make adjustments to his plans, responding to the board's concerns.
Palo Alto city planning director Hillary Gitelman told CNN Money once those adjustments are made, his team will request that the board will reconsider their recommendation.
She said they are hoping to schedule that meeting to take place in November.
