The only thing to establish is IF your baby has downs how likely it is that they would need additional medical assistance immediately at birth. Take that chance, multiply it by the 1/100 chance you already have and you have the risk factor for this baby.
We had been told that our baby had a potential kidney problem that would require treatment from birth. In theory this would mean no home birth but when we actually asked for the details it just meant that she should start taking antibiotics within the first 24 hours after birth until they established how badly she was affected. Once we knew this we just carried on and had her at home, knowing that if she needed it we could take a trip to the hospital within the first 24 hours for treatment. As it happened she was fine, but we had plans of action either way and neither involved having to birth in hospital.
Equally, I had a doula client who went into labour at 35.5 weeks. She knew there was a chance her baby would need additional support due to being early but the midwife who cared for her - that we knew and trusted - said that babies at that gestation are usually fine immediately at birth, but if they're not quite ready will start to get tired and need support within a few hours. They had a beautiful home birth and then when their baby did need some help, they transferred in when he was 2 hours old. He spent a week in special care which was really tough for them, but at least they had their wonderful birth to look back on and remember fondly.
These are both examples where potential problems for the baby were considered, understood, but ultimately were deemed by the parents (not necessarily the medical professionals - there was quite a bit of negotiation in both cases) not to be sufficient to merit a hospital birth 'just in case'. There would be time in both cases to transfer if it was needed without major risk. I don't know enough about downs to know what risks you might face, but for me at least, that is what it boils down to.
1% is a very small percentage. Only you can decide how much of an impact you're willing to let it have, and you need more information before you can make an informed choice. Good luck with it!
Gina. x