I used to work for a housing association that built homes for shared ownership. Things might have changed since I gave up work, but if they haven't, I would not recommend this type of tenure for anyone unless they had no other options.
This is how it was then (and might still be):
Shared owners enjoy none of the benefits of home-ownership while shouldering all the disadvantages.
- You are still a tenant, but
- You pay full market rent on the portion of the property you don't own and
- You are responsible for paying the full service charges, including on the portion if the property you don't own.
- You are entirely responsible for short-term, long-term repairs and improvements. Including on the portion of the property you don't own.
- You are likely to be limited in any changes you want to make to your home while it is part-owned by the housing organisation.
You also have none of the benefits of being a tenant such as responsive and maintenance repairs and general improvements carried out without any additional cost to you. Nevertheless, you will be expected to carry out and pay for anything that needs doing in a timely manner. The housing organisation will take swift enforcement action against you if you allow upkeep to slide (for example, if you don't have enough money to pay for work).
If at some stage your financial status improves, you can buy more of the property. However, don't expect to buy it all at once. Many (if not most) housing organisations only allow you to buy in bits. They call it staircasing. You will probably be limited to buying in several stages (25% of the value at a time, for example). Each step is a long and costly process and you will have to pay the housing organisation's legal costs as well as your own at every step. You will then have to wait a set period of time before you can buy another "step".
This means, if the housing organisation uses 25% per step staircasing and you buy 50% of the house at the start followed by two staircasing "steps", you will go through three purchasing stages and have to pay six lots of legal fees (yours and theirs) before you become homeowners.
At each step of the staircase you will have to share any increase in equity with the housing organisation, so you would pay 25% of the market value at the time you "staircase". If the value of the property goes down you will still have to pay 25% of the original purchase price for each step so the housing organisation doesn't lose out.
I would definitely suggest you look at other options (cheaper area, smaller house) if you can. This is a very expensive way of buying a house.