Depending on how far away your remote building is from your main house, and the density of obstructions between the two buildings (e.g., trees, sheds, RV, etc.), there are a couple solutions. The overall cost depends on the scenario below:

Scenario 1:

The main house and the remote building are close together (<50 ft), and neither structure has a metal or stone exterior (e.g., aluminum siding, sheet metal, brick, cinders, etc.), and there are no obstructions between the two buildings (e.g., open space between the two).

Solution:

Buying a more powerful wireless router for your home or purchasing a wifi booster (also known as a range extender) and placing it at the wall nearest the remote building, might extend the wifi signal from your router in the house to your remote building.

Scenario 2:

The main house and the remote building are farther apart (>50 ft), and there is a clear line-of-sight "window" between both buildings (e.g., you can see the remote building from the main house).

Solution:

A 5 GHz Point-to-Point (PtP) link between the two buildings would work well. The two buildings can be up to a mile apart, as long as they have a clear line-of-sight to each other.

This solution requires the addition of a PtP antenna at/near your main Internet antenna, and installation of a second PtP antenna at the remote building with cabling to a second router or access point. Costs are about $350 for parts and labor (may vary depending on your circumstances), plus the cost of an additional router. This is a one-time cost and the Internet is shared between the two buildings.

Scenario 3:

The main house and the remote building are farther apart (>50 ft), and/or there is considerable vegetation between both buildings (e.g., you cannot see the remote building from the main house).

Solution:

A 900 MHz Point-to-Point (PtP) link between the two buildings would work well. 900 MHz is the only frequency that works through trees. The distance between the two buildings depends on vegetation density (e.g., less vegetation density means the building can be father apart).

This solution requires the addition of a PtP antenna and PoE switch at/near your main Internet antenna, and installation of a second PtP antenna at the remote building with cabling to a second router or access point. Costs are about $650-$850 for parts and labor, plus the cost of an additional router. This is a one-time cost and the Internet is shared between the two buildings.