Building an outdoor arbor or pergola is a project that can be scaled to fit almost any budget. Pergolas can be constructed to cover pathways that span a hundred feet or more, or used as small dividing fences between raised beds in your garden. An arbor can be over a small patio with a single seat, or be constructed to cover a hot tub or elaborate seating area. There are a few factors that will help you estimate how much your pergola or arbor will cost to build.
The most obvious question you need to ask when building a pergola or arbor is what it will be used for. If you just want a quiet place to sit in a porch swing, the construction costs will be substantially lower than if you intend to use the structure to entertain or cover a hot tub. A good rule of thumb for outdoor construction such as pergolas and arbors is to expect to pay £6 per square foot, if you are doing the construction and assembly yourself.
Depending on what sort of climbing plants you intend to grow on your pergola or arbor, you may be able to reduce costs by using smaller amounts of wood. A grapevine is about the heaviest thing you can grow on such a structure. With a mature vine in fruiting season, you will need to use nothing less than 2-by-4s for the overhead latticework, and 5-by-5s for the horizontal supports. A climbing rose, on the other hand, is substantially lighter weight. Overhead latticework for a rose can be a simple length of wire or vinyl lattice supported by 2-by-2s.
Besides the construction of the actual pergola or arbor, you must consider what sort of flooring to put in. The most expensive option, at about £6 per square foot all by itself, is gazebo wood flooring. This will get you up and off the ground, but it requires a higher ceiling overall, and thus inflates construction costs. A cement slab is a good way to go if you are growing grapes, as the fallen over-ripened fruit can easily be washed off with a hose. Another option may be just bark nuggets or mulch, which cost as little as 60p per square foot.
Depending on the location, you may need to beef up the horizontal supports and vertical latticework to account for high winds, or heavy snowfall in the winter. Pressure treated lumber is typically about 50% more expensive than its non-treated counterpart, but can withstand more abuse without cracking or splintering. Additionally, if termites or carpenter bees are a problem in your area, it may be worthwhile to look into staining or painting the wood to prevent rotting.