Welcome to BYC and the wonderful madness of chickens.
https://www.backyardchickens.com/threads/helpful-hints-for-building-coops.1191089/page-763 has become more of a chat thread, but there is a wealth of information if you want to skim through back posts.
Go right ahead and start a thread asking for general advice if you like. We don't mind.
Here's a little except from an article I'm working on that might help you:
For each adult, standard-sized hen you need:
1 square foot is 1 foot wide and 1 foot tall. Or 2 feet wide and 6 inches tall. Or 4 feet wide and 3 inches tall. Or a triangle under your gable peak that is 2 feet wide at the bottom and one foot tall.
- 4 square feet in the coop,
- 10 square feet in the run,
- 1 linear foot of roost,
- 1/4 of a nest box,
- And 1 square foot of permanent, 24/7/365 ventilation, preferably located over the birds' heads when they're sitting on the roost.
If you're measuring odd sizes in inches you can convert your square inches to square feet here.
Here are some things that are about 4 square feet:
Bathmat, 4.5 square feet
Doormat, 3.9 square feet
Leaf for dining table, 3.5 square feet
3 seed starting trays, 4.1 square feet
Since the minimum advisable chicken flock is 3, you need at least 12 square feet, approximately the size of a pallet. So no coop smaller than 3x4 is actually appropriate for keeping any full-size chickens at all.
This, unfortunately, demonstrates just how inadequate many prefab coops are since many of them are smaller than that despite claiming to house far more than 3 chickens.
The 10 square feet of run space might be a little easier to visualize. 2'x5' or a little more than 3'x3'.
Here are some things that are about 10 square feet:
My desk, 9.6 square feet
A large bath towel, 9.1 square feet
Footprint of guest bath/shower combo, 10 square feet
Back bedroom window, 11.9 square feet
For that same minimum advisable, 3 chicken flock you'll need at least 30 square feet in the run. That's 4'x8', or 5'x6', the approximate size of a queen mattress or a modest walk-in closet.
Therefore, if you're wanting 3 chickens and are looking at buying a coop and run you can visualize the size of a pallet next to a queen bed. Or 3 bathmats next to 3 bath/shower inserts. If that won't fit, the coop is too small.
Some More Numbers
Height can't compensate for lack of floor space because chickens don't stack for storage, but there are some useful things to consider about height.
First, the base of the coop needs to be deep enough to hold a reasonable amount of bedding. "Reasonable" is a flexible term , but 4-8 inches is a good starting figure. If you're scooping poop daily you only need enough depth to hold sufficient sand/Sweet PDZ/etc, to keep the poops from sticking to the floor. If you're doing Deep Bedding or Deep Litter and want to clean only a couple times a year you might want room for a foot or more of bedding.
Second, the nest boxes, which are usually 12-16" cubes, need to be above the maximum top level of bedding to keep the clean nest lining from being contaminated by the soiled bedding or you will get poopy eggs.
Third, the roost needs to be above the nest boxes so that the chickens will sleep on the roost instead of in the boxes.
Fourth, the ventilation is best placed above the chickens' heads when they're sitting on the roost -- 8-12" depending on the size of the hen and her comb type.
So, the minimum height to get things stacked up correctly is between 3 and 4 feet high, depending on the slope of the roof and exactly how the ventilation is arranged. Additionally, chickens aren't helicopters so they usually fly up and down to their roosts about about a 45-degree angle. That means that the roost should be at least as far from the wall at the end of their landing zone as it is high.
Additionally, many people like to have small coops raised up both so that the chickens can use the area underneath them and so that they can clean more easily, sticking a wheelbarrow or garden cart under the access door and pushing the bedding out instead of forking it up from ground level. 16-18" is probably the minimum to allow full-sized hens to walk around with a little bedding underneath to absorb the poop while much over 24" will become challenging to clean except for the tallest of chicken-keepers.
A useful article on heights: https://www.backyardchickens.com/articles/coop-stack-up-how-high-stuff-works-well.73427/
Since lumber is sold in multiples of 4 it's actually more practical to make a 4x4x4 coop than a 3x4x3.5 coop. At 4'x4'x4', my Little Monitor Coop is designed to meet all the minimums for a flock of 4, standard-sized hens.
You might also find this helpful, I revisited my newbie questions a decade later: https://www.backyardchickens.com/articles/my-newbie-questions-10-years-later.76713/
Where, in general, are you located? Climate matters, especially when building your coop.
I can recommend a number of other articles dealing with cold weather, heat, space recommendations, and more as applicable to your situation.
