When you are digging the hole for the foundation, that is the start of building your home, barn or whatever building.
Most of the time, you will have to dig into the ground to place the footings of your foundation. Hire an excavator who has the right sized equipment for the job. Hiring a too-small excavator with a low hourly rate could end up costing you much more in the long run.
You will need to excavate an area based on your building size and location on your property as per zoning bylaws. When you are digging, you should over excavate the hole by a few feet to make sure you can efficiently work around the foundation. You will often have to install a perimeter drain, and you will need room to do that too. Much cheaper doing it right the first time than having a machine come back afterwards.
To get the hole in the right place, you need to locate the corners of your building and extend outward from these corners. Depending on the height of the foundation, over excavating by 3 feet all around the foundation will be a minimum. You have to use common sense. For ease of work, and more importantly, your safety, and at times, you need to over excavate by much more than three feet. Discuss your needs and concerns with the person you hired to do the excavation to ensure you end up with enough space to do the work and that you can do it safely.
For certain situations, you could dig trenches where your footings are located instead of excavating the whole area. Trenching will work great for simple layouts. But if there is plumbing located underneath the slab, you will create a lot more work for a plumber. And Plumbers don't like digging any more than they have to! Excavating the whole area nicely and level, stepped if needed, makes building your foundation and the additional work you need doing inside the foundation much more straightforward. So suck it up and don't get cheap on the excavation.Calculator: Estimate how many truckloads it will take to remove excavated material off-site.
Start with removing the layer of topsoil, if there is any. Place this in a separate pile out of the way and keep it uncontaminated from other materials. Topsoil is expensive, and you don't want to use this type of material under your slab. It makes a lot of sense to save it and use it for your lawn or garden areas.
You usually end up keeping some or all of the excavated material on-site. Keep the better material, and I know it takes time, separate, sorted into material that would be nice to use inside the foundation and a pile of material that you can use to back fill the outside.
Pile the excavated material around the hole for faster digging. Or not! That has to be one of the dumbest things you can do when excavating. If you are not using a surveyor to pin your building, most likely, you will need to use at least one of the rear and front corner pins to get your setbacks. Keeping one side clean makes a whole lot of sense! Keep an area in front clean too, much easier to get the materials in the hole.
And also, consider, while having an excavator on-site, digging the area where the sewer has to go. The connections for sewer and water lines are usually deeper in the ground, and it will not hurt to make your other services connection at the same time. If there is a gas line going to the house, don't worry about digging that out. The gas company will take care of that after the final grading of your lot. Note the exact locations of any service lines you install on your plot plan and follow the motto, locate before you dig!
In most cases you will require the services of a surveyor. If you do, consider getting your surveyor to come to the construction site three times. One time to locate and mark the corners for the excavation. A second time is for locating and marking all the required corners of your foundation and a third time for the official survey certificate. It will cost you a wee bit more, but you will sleep much better! Out here, that is considered a package deal that you can get from your surveying company.