The cost to build a 20 x 20 shed by doing it yourself would cost between $2,000.00 to $3,000.00 depending on the materials you buy.
If you want a concrete floor then the cost would be a bit more and if you want a wood floor or dirt floor then the cost would be less.
Hiring a shed builder to build the 20 x 2 shed would cost around $5,000.00 to $10,000.00 depending on who you hire to build the shed.
When building a shed close to a property line most counties and towns have a rule about how close you can build to the property line.
This is called a setback line which means you must leave sufficient room between your property line and the neighbors property line when building the shed, house etc.
Most places have a rule that says that when you build something you must build it a minimum of 5 feet away from the property line.
However some places have the set back line as 10 to 15 feet.
You would have to check with your local building codes such as at the town hall about the actual and correct set back line.
But most of them require you to build your shed, house, etc at least 5 feet away from your property line.