Island Depth
When calculating island depths with seats, don't forget the counter overhang in front of the cabinets (non-seat side) - usually 1.5"- and the thickness of an installed decorative door or end panel on the back of the cabinets (the seating side) - usually 1".
If you have cabinets on both sides (so doors open into the seats, then that 1" is for the cabinet doors.
So:
1.5" counter overhang in front of the island
24" depth for each standard base cabinet -or- whatever depth you're using (some people have 24" for the front cabinets and then only 12" or 15" for the cabinets on the back that open into the seats)
1" for door or decorative door/end panel
15" overhang (counter height)
With just one set of cabinets, the minimum depth of the island is 41.5"
1.5" + 24" + 1" + 15" = 41.5"
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Island Length
For island length - you need at least 24" per seat.
Edited to add: As FunkyCamper points out in a later post, this is the minimum width per seat. There have been some rumblings about an update of the recommendation to 30" per seat, but as far as I know, that hasn't happened yet. With 5 growing children, I suspect 30" wouldn't be a bad idea - it will help minimize arguing about "s/he's in my space" or "s/he is touching me". In addition, if your children are taller/bigger than average, the extra space would be nice once they hit the teens.
If you have seating on two sides, remember that two seats cannot share the same knee/leg space in the corners!
For an island that seats 5 to 7, I strongly recommend seating on at least 2 sides, 3 would be even better! You don't want to have all 7 (or even 5) seats sitting like ducks in a row. Straight-line seating for more than 2 or 3 seats is not very conducive to conversation and bonding - it's more like dining with strangers in a diner.
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Aisle Widths
On the working side:
One-person...You need a 42-inch aisle if only one person will ever be working on the aisle (I assume that will not be the case with such a large family - some children will be helping out/learning to help out)
Two or more people working on the aisle (most likely scenario in this case)...You need a 48-inch aisle
Aisle behind the seats - depends on what else is behind the seats
With a wall or other obstruction behind the seats:
If no cabinets, counters, appliances, and no traffic at all (i.e., "dead end"), then 44" is sufficient
If no cabinets counters, appliances, and minimal traffic, then 48" is sufficient
If no cabinets counters, appliances, and normal to heavy traffic, then 54" is sufficient
If cabinets counters, appliances, etc., are behind the seats, then 60"
No wall or other obstruction directly behind the seats - the "open" concept your friend is planning:
If island backs to a sofa or similar - then no less than 48" if there is minimal traffic (i.e., a "dead end") or 54" if there is normal to heavy traffic.
If island backs to a table - then 60" b/w the island and the table
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Above window idea
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