The first thing you need to decide is how big of a building do you need to build.
First you have to figure out what purpose this building is going to serve and how much square footage you need for your new building.
Depending on where you live, it may be easier to get permits for a building under a certain square footage. Around here, the permitting process is much easier for buildings under 120 square feet (10′ x 12′).
The easiest way to figure out how much space you need is to get your tape measure out and mark off an area in your yard.
Let’s say you start with an 8′ x 10′ area. Will all the stuff you want to have in your building fit in this space?
Make sure you leave enough room to get around in there without having to move all your stuff around to get at something.
Build for purpose
If your building is going to serve another purpose, like your home office, a workshop or a backyard getaway, make sure you plan the size of the building for all the furnishings you will require. Desks, couches, work tables, cabinets, woodworking machinery, etc… should all be taken into consideration when determining your building size.
The difference in the cost of materials between an 8′ x 8′ and a 10′ x 12′ building is not that much considering that the 10′ x 12′ is nearly twice the square footage. (64 sq. ft. versus 120 sq. ft.) Everybody can always use more room. Plus, if you are going to be constructing this building by yourself, you are going to save on the building labor costs. So reinvesting these savings into more space is a good idea.
Adding on to your building later on
While it is possible to add on to these buildings later, by adding a “shed roof” or lean to style addition off of the side(s) to store yard stuff like lawnmowers, bicycles or firewood, increasing the floor space on the interior later on is a different ball game.
If you are serious about possibly expanding the interior space of your building, plan for this by increasing your wall height from the start. This will make it easier for you to add additional storage space going forward. If the walls are higher, you can easily tie into the building (below the existing roof) with simple lean-to or shed roof additions later on without having to compromise the headroom that will be available in these future additions.
Increasing your wall height will change the standard measurements as outlined in the guide. Just keep that in mind if you stray from the standard layout.