It would be best to get a couple of different builders in to have look and give you a rough quote before you get an Architect involved.
By doing this you can explore your options before committing to anything as well as seeing the price difference between the companies for the same job.
The conservatory should not be an issue as the scaffolding company appointed by yourself or your builders will raise a platform above it to make sure it is safe and access can still be achieved for workers.
