WoooooHoooo Nate to the rescue, Thanks my friend...
There will be no need to raise the floor, but those holes will have to be covered or fixed.
That big door on the side will need to be closed and secured so it can no longer be opened, then "if possible" cut out for 2 windows on that door, storm windows can be found fairly cheap at garage sales and or in the paper, or craigs list, measure what size will be needed, either add one big window or 2 small ones, the will allow plenty of light in and a good soarce of ventilation, 2 major keys to healthy birds.
Also if possible, a small window in the door in front, this door will be used to enter and exit, if you want to palce a lock onto the door...
Standing in front of the shed looking at the door...look up where the roof comes to a peak, this is where you will cut a six wide by 8 inch high hole, square at the bottom, round at the top, this will be where the birds come and go from.
Under the hole on both inside and outside place a 6 inch wide shelf, a "landing strip", the birds will need something to land on while entering there loft, this landing strip can go from one side of the roof to the other as far as length, or 10-12 inches will be fine.
I use both types of perches, a 1 by 6 shelf and individual perches so the birds can have there privacy. here's a picture of the perches that only allow one bird to perch, this controls fighting also as only one bird can perch.
these perches should be mounted as high as possible.
Example picture

You can mount these single perches where ever within the shed, birds like to perch up high, but not to close to the ceiling as to where they can't land comfortable, I will say atleast a foot down from the ceiling.
Even a 1 by 6 inch shelf level with the bottom of the windows so they can look out while perched will be fine...
Water and seed dishes, I use cat & dog water dishes, the kind that come with a gallon water jug, but a regular heavy ceramic bowl will work, and change water daily, the seed bowl should be placed away from the water soarce as pigeons are flingers, they love to throw seed around, so otherwise you will have the water bowl full of seed....
Its near impossible to keep clean water, so do your best to change the water if they poop in it....
Hope you don't mind cleaning, get youself a scrapper, metal...heavy duty and plan on scraping and either vaccuming or sweeping the floor once a week, avoid placing the seed and water dishes directly under a perching area.
Once you have completed your loft, you will want to train Abi to use the loft, place seed on the landing strip just inside of the entrance and maybe a little bit outside so she can see the seed and start there.
Arty