I've done this for a cottage and a garage loft on the Cape and two second floor additions for cottages on a lake in Maine.

First make sure the upper floor joists are adequate. they are usually sized to only support a ceiling. Measure the span, spacing and size of the members and let us know.

Then you need to draw the floor plan carefully to scale.

For the stairs, measure the floor to floor height in inches (including any flooring changes), divide by 8.25" then round up to the nearest whole number of risers. Subtract one from this number to get the number of treads. Multiply this by 9" and convert to feet & inches which is the total length of the stair not counting the landing dimensions top and bottom. Assume the stair will be 3 ft wide and the landings 3ft x 3 ft.

It is also possible to use an alternating tread stairway which cuts the stair length in half but might be difficult for small children to climb.

If the upper floor area is under the eaves of the roof rafters it is a good idea to put as much of the stair as possible in the low ceiling area and have it rise as the roof increases in height. That may be difficult to do on the lower level but give it a try. It helps if there is a hall or entry foyer available so you can enter the stair without creating another hall space. A stair that turns 90 degrees half way up sometimes fits better in the lower level plan but it takes up considerably more space on both floors.

Beadboard paneling is available in 4x8 sheets for paint and in 3/8" thick t&g pine strips for clear finish.

If this renovation is done with a building permit you will need to provide one Emergency Escape window in each separate bedroom and provide a code set minimum level of light and natural ventilation. Wiring will have to meet the NEC. Insulation of a minimum R value of 3/inch must fill the rafter and knee wall cavities with vent baffles added between the rafters. A vapor retarder should be installed behind the new wall and ceiling finishes. The roof will need vent openings added at the eaves and the ridge. The stairway will need a handrail on one side and a "guard" 36" high where the upper floor or a landing is more than 30 inches above the lower floor.

Wow. Great information! I hope the contractor that lands the job knows all this! Is this MA code or general?

It's from the 7th edition of the Massachusetts One and Two Family Dwelling Code which is available online at the link below. Most of the relevant design requirements are found in last third of Section 53 "Building Planning for Single and Two Family Dwellings" which can be downloaded as a separate PDF file from the website.

Are you going to hire a contractor to design and build it, or design it yourself (with the help of a designer) and then bid the drawings competitively?

Here is a link that might be useful: MA one and two family building code