Nicola,
If you want to convince your Mum, then you need to see her point of view first.
She has valid points. Believe me, they're valid points! lol
How many birds are you planning to have?
When the chickens are full grown, you won't believe the amount of fertilizer they can produce! I only have four standard hens, so it's a bit easier to keep the smell, flies and droppings cleaned up. If you'll get into the habit of taking care of the fertilizer (where will you put it???), then you can help to calm your Mother's fears.
I clean out the coop (droppings and most of the pine shavings) every Saturday. These go into an area in the back of our garage and are composted. I've got a 14'x6' run attached to the coop. I throw grass clippings into the area, and sometimes greens from the kitchen. The girls will scratch the area down well, and it looks just like plain dirt. Fluffy dirt, but plain dirt. Once in a while, the run can have an 'earthy' smell, but it's not anything to complain about. I rake/shovel out the top couple of inches every few months and add it to the garden. The hens freerange in the backyard from noon until about 5pm every day. And it's a mess! I am good about cleaning up after them every day, but I'm tired of using water to hose off the bricks, etc. And I walk around the grass and shovel up the chicken droppings so that no one will walk in it and track it into the house.
I'm going to build a little fence that will let them free range out of the run, but still contain them where people won't be walking.
Have you kept your bedroom clean? Do clean up after yourself in the bathroom (no wet towels, toothpaste cap left off), and put away your dirty dishes in the kitchen? If so, your Mother is more apt to believe you that you'll keep the coop and the chickens' area clean.
Carla