What affects the cost of a deck

Size

Bigger costs more, but not in a straight line. Set-up, footings, the ledger against the house and the edge details are roughly the same whether the deck is 12 or 20 square metres, so the cost per square metre usually falls as the deck grows. A very small deck is the most expensive per square metre you will ever build. Odd shapes, angles and curves add cutting and waste on top of area.

Height

Height is the factor people underestimate. A deck under 500 mm sits on stumps in the sand with simple bearers. Once it is over 500 mm it may need planning approval in either council, it needs privacy screens within 7.5 m of side and rear boundaries, the structure moves to posts and bracing, and if the fall is 1 m or more the National Construction Code requires a balustrade. Stairs come with it. A deck that is 900 mm off the ground can cost far more than the same deck at 300 mm, and none of that money is visible in the boards.

Boards

Hardwood and composite are priced differently and behave differently.

Merbau is the everyday decking hardwood and generally the least expensive of the three timbers we build with. Spotted gum and jarrah cost more per board. All three need oiling over their life, and jarrah greys fastest without it.

Composite boards usually cost more than hardwood per board, and the hidden clip systems most of them use add to that. Against that, composite is the last money you spend on the surface: no oil, no sanding, no replacing split boards.

Board width and profile matter too. Wider boards cover faster but cost more each. Boards laid on the diagonal look good and use more timber and more joists.

The frame and fixings

The frame is treated pine or hardwood on stumps or posts, and its cost tracks the height and the joist centres. Composite needs tighter joist centres than hardwood, so a composite deck has more joists in it. On the coast the fixings are stainless steel, which costs more than galvanised and is not optional within reach of salt air.

What is in the ground

Most of the corridor from Joondalup to Two Rocks is coastal sand, and a standard stump footing goes in without drama. In parts of the far north limestone sits close to the surface, and footings there are core drilled, which takes longer and needs different gear. The contractor will know within the first hole, which is why the quote follows a site visit.

Access

Everything for a deck arrives at the front of the house and has to get to the back. A side gate wide enough for a wheelbarrow is fine. A deck at the rear of a narrow lot with no side access, where every board and bag comes through the house or over a fence, takes longer, and time is cost.

Approvals and engineering

A building permit application needs drawings. A raised deck usually needs engineering. Planning approval, where a deck needs it, has its own fees and its own drawings. These are real costs that sit outside the build itself, and a quote should say clearly which of them it includes and who lodges what. Our approvals guide explains how each council treats decks.

Removing what is there

Taking out an old deck, a concrete slab or paving before the new deck goes in is a separate line in the quote. Disposal of treated timber and concrete costs money. Termite damaged timber is handled and disposed of with more care again.

Extras

Balustrades, stairs, privacy screens, lighting, a skirt around the underside, a drainage membrane under a raised deck, a picture-frame border, an inset for a spa. Each is a good idea somewhere. Each is a line on the quote.

What to do with all this

Decide the height and the board first, because they set most of the cost. Then get the site looked at. A written quote from the contractor, after a site visit, is the only number worth planning around. Get a free quote and the contractor will call to arrange one.