Whether you are designing a new house or designing a new deck, one of the first steps is to create a building plan. Building plans aren't drawn to size, they are drawn to scale, which means that you "shrink" the area down and make a scale model on paper. How much to scale down depends on how large the plan is, how detailed you want those plans and how big your paper is.
Measure the longest part of your building project, in feet. For example, the projected length of your house is 10 metres (30 feet).
Measure the longest part of your paper, in inches. For example, the length of your paper is 25 cm (10 inches).
Divide step 1 by step 2. 10 metres (30 feet) / 25 cm (10 inches) = 0.4 metre/cm (3 feet / inch) This means that every 1 cm (1 inch) of paper space represents 0.4 metre (3 feet) of "real" length.
If you have an unusually long building plan, you may want to consider purchasing a roll of paper, which you can cut to size for your plan, rather than having half of the paper space empty. Add a few metres of length around the edge of your building project if you want a border around the edge. Use your scale drawing to calculate area by converting to "real" measurements before multiplying length by width. For example, if on paper, an area is 13 by 13 cm (5 by 5 inches), then 13_0.4 = 5.2 metre (5_3 = 15 feet), so 5.2 by 5.2 metre= 27.04 square metres (15 feet by 15 feet = 225 square feet).