Hostel design

1. There will be some double/single rooms, however due to some constraint, there will be some rooms without a windows. How will the general feeling about this? Is this acceptable?

2. Do you consider a hostel without a dorm but all double/single rooms?

I've been to one hostel that didn't have any windows. It was in Athens, Greece in the middle of the summer and was very hot. From what I've seen, if backpackers have a great time at a hostel, they will often forgive other shortcomings of a property.

Hostels are generally shared accommodation, but you could still market it as "backpacker accommodation" and attract the same types of travelers.

If there are no shared rooms, then it is a hotel and not a hostel.

I was thinking maybe a hybrid would be a good idea - like along the one side you could have a dorm, with a good few beds in it, then along the other a few basic en-suites. Obviously you'd charge more for the en-suites.
Basically offer two types of budget accomodation in the same place thus doubling your potential client base. :D

I was thinking maybe a hybrid would be a good idea - like along the one side you could have a dorm, with a good few beds in it, then along the other a few basic en-suites.

That is a great way to do it... Many people stay at hostels just for the social atmosphere, even though they can afford to stay somewhere more expensive. (I even saw a hostel guest pull up in a rented Ferrari once.)

The private rooms at some hostels are more expensive than the local hotel rooms and they still sell out. Some people like the social atmosphere of a hostel but still want their own room. Or you sometimes get school groups where the students stay in dorms and the leaders want private rooms...

Two of the hostels I worked at where they had a mix of privates and dorms had systems for converting rooms from one to the other, depending on demand.... generally some extra bunks in storage and a way to quickly convert a room from one type to another.