Sold by the square metre, but paid at the perimeter and by the height

The deck is the garden work with the apparently simplest price: so many square metres, so much per metre. Then the real quote arrives and the numbers stop adding up, because the two commanding items do not live in the area: the perimeter — railing and stairs scale with the edge and the height, and on the small deck they are worth as much as the floor — and the level: the same surface at ground level on ready soil or suspended on posts over a drop are two different works with the same name. This calculator breaks the quote into its three forces — square metres, site, perimeter — and sums them up in two honest numbers: the effective square metre and the index ι, how many times the total exceeds the metres-only bill.

The formula: square metres, site and perimeter

The model is the decomposition of the light worksite. The metres bill is A·c: the area times the cost of the complete package — boards, substructure, fasteners and installation — for the chosen species. On that bill acts the site surcharge s: zero at ground level, above 40% when the drop demands posts and framing. Outside the product stays the perimeter P: railing and stairs, the item that does not grow with the area. The index ι = C/(A·c) is dimensionless and works in any currency: it says at once whether you are buying floor (low ι) or a small structure with framing and railings (high ι).

The scale of the area: the perimeter dominates the small deck

The table grows the deck with everything else held equal (180 per m², site 20%, perimeter 1,500):

On the 5 m² balcony deck the perimeter is worth more than half the price and the effective square metre is nearly triple the list; on the large 80 m² deck it disappears into the bill. It is the same geometry as the fence's gate: the fixed cost disguised as an accessory — and the list-price comparison does not tell it.

A complete numerical example

Typical deck: 20 m² in good-quality wood at 180 per metre installed, a site with a modest drop declared at 20%, railing on two sides and a small stair for 1,500. The metres bill is 20 × 180 = 3,600; the site brings it to 4,320; the perimeter closes at 5,820. The index is 5,820 / 3,600 = 1.62: the effective square metre is 291 — not the list's 180 — and the perimeter weighs 25.8%. Whoever compares two quotes «at 170 and 190 per metre» without reading site and perimeter is comparing the wrong item.

How to read the quote index

The four bands tell four worksites. Below 1.2 the quote is in line: ground-level deck, ready soil, minimal perimeter. Between 1.2 and 1.5 the inflation is contained: the physiology of the well-quoted deck. Between 1.5 and 1.9 site and perimeter make themselves felt: it is survey territory, where the off-list items are a third of the price. Above 1.9 the floor is only the beginning: raised framing, code railings, stairs — the deck has become a small structure. No band is «wrong»: the index says where to point the lens before signing.

The cost per metre: what it really contains

The variable c sums up the species choice, and the range is nearly an order of magnitude wide. Pressure-treated pine lives in the low band: cheap, honest, asking maintenance every two-three years. Thermally modified woods rise: stability and durability in exchange for price. Exotic species — ipe, teak — sit high: density, twenty-year life, more demanding installation. Composite (WPC) plays another game: it costs like a mid-high wood but nearly zeroes the maintenance. Inside c also live the substructure, the stainless fasteners and the installation: the «boards only» catalogue price is typically half the installed metre — the twin mistake of the perimeter one.

The site: the item that turns the deck into a stilt house

The surcharge s is what separates the floor from the structure. The drop is the queen item: every step of height demands posts, sized beams and — above certain heights — the railing by code, which re-enters through the perimeter's door. The ground counts as much as the height: the ground-level deck wants a stable, draining plane with a weed membrane; soil that gives way is fixed first, or the deck waves with time. The access and the obstacles — the tree to build around, the cut corner — multiply the cuts and the hours. The working rule is one: the site is declared at the survey, because it is the only item you can only see by walking on it.

International perspective: square feet and the same three items

The deck is universal, the conventions are not. North America — the deck's homeland — prices by the square foot (multiply by 10.8 for the square metre) and mandates railings above height thresholds set by local codes; Europe prices by the square metre with the same items. Everywhere the bill has this model's three forces: area, site, perimeter — and everywhere the same code rule holds: above a certain height the railing is not a choice, and its exact threshold is set locally. The calculator is parametric and currency-free precisely for this: the index ι compares quotes from different markets on the same scale.

Limits of the model and correct use

The model photographs the standard deck: it does not cover the pergolas and roofs that often accompany it, the integrated lighting, nor the building permits where heights require them. The area is measured on the real drawing — L-shapes and cut corners consume more material than their area — and the perimeter is specified item by item: railing per metre, stairs per step. For the garden's perimeter the fence cost calculator does the twin count with the same disguised-fixed-cost logic; and if a tree must be removed from the footprint — or wisely built around — the tree removal cost calculator closes the picture.

The scale of the site: from the lawn to the stilt deck

The surcharge s is the item that separates two quotes with the same boards. The table runs it over the base case (20 m² at 180, perimeter 1,500): at 60% the model changes register — from there on, the level commands the quote.

Between the ready lawn and the drop demanding posts run 2,880 — sixteen square metres of deck at list price — without a single board having changed: what changed is what lies underneath. That is why the serious quote is born from a walked survey: the site is the only item you can only see by standing on the footprint — the height, the yielding ground, the tree in the middle.

The scale of the perimeter: from the free edge to the balustraded deck

The perimeter P grows with the exposed sides and with the height, not with the area. The table raises it on the base case (20 m² at 180, site 20% — gross 4,320):

The zero row is the ground-level deck with no obligations: a clean 1.20 index, the pure floor surcharged by the site. At the other end, at 5,000 the perimeter is worth more than half the price: it is the three-sided balustraded deck with stairs — and the right question is not «how much does the railing cost» but «where is it truly needed?»: one side against the house is free perimeter, the stairs consolidated in a corner halve the steps.

The effective square metre: the number that compares quotes

The list says «180 per square metre», but no deck pays the list: it pays the total. The effective square metre — total divided by area, site and perimeter included — puts the quotes back on the same line: 291 in the base case, 516 on the balcony deck where the perimeter dominates, 235 on the large one where it dilutes. It is also the number that unmasks the teaser price: the «160 per metre» that excludes the substructure or counts the stairs apart ends up, at the end of the job, above the «200 per metre» of whoever had written everything down. Totals are compared at equal specification, never the first line.

The invisible half: substructure, ventilation, slope

The difference between the deck that lasts twenty years and the one that rots by the fifth sits almost entirely under the boards. The joists sized and spaced to the board thickness; the pedestals or footings that lift the wood off the ground; the ventilation — the true secret: air must pass underneath, or moisture works from below upwards; the 1-2% slope that drains the water without being seen; the stainless fasteners that neither stain nor give. It is the reason of the recurring ratio «installed = 2 × boards»: the second half is not seen, but it is the one you truly buy — and the item where the cheapest quote has typically cut.

The deck's life: the square metre per year is the true price

Two decks with the same quote can cost double one another over the life cycle. Treated pine starts low but asks oil every 2-3 years and lives 10-15; exotics start high and live 20-25 with nearly aesthetic maintenance; composite pays everything upfront and then only soap and water. The mature comparison is made on the square metre per year: installed price divided by the expected years, plus the cyclic maintenance — hours or invoices that belong in the count. The «expensive» species often wins: the calculator's effective metre photographs the purchase, the yearly metre photographs the choice.

From the count to the design: the four decisions before the wood

The index says the quote; the design decides the rest. The height: every centimetre above the code threshold buys mandatory railings — the lower deck is often the smarter one. The board orientation: perpendicular to the main view they look tidier, but the right direction is decided by the framing underneath. The shapes: the L and the cut corners consume more material than their area — the rectangle is the king of yield. The future services: the socket, the light and the conduit are laid before the boards, not after. The quote signed on these four decisions, plus the index, is a deck that is not renegotiated with the site open.

Frequently asked questions

How much does building a deck cost?

With the decomposition C = A·c·(1+s/100) + P: the square metres times the cost of the complete package, surcharged by the site, plus railing and stairs. With 20 m² at 180 per metre, site at 20% and a 1,500 perimeter, the total is 5,820.

What is the deck quote index?

It is the ratio ι = C/(A·c) between the total and the square-metres-only bill. Below 1.2 you are buying floor; between 1.5 and 1.9 site and perimeter weigh a third; above 1.9 the deck has become a small structure — framing, railings, stairs.

How much does an installed deck cost per square metre?

The species range is wide: pressure-treated pine in the low band, thermally modified woods in the middle, exotics and composite high. The number to compare is the effective square metre — total divided by area, site and perimeter included: in the base case it is 291 against the 180 of the list.

Why does the railing weigh so much?

Because it scales with the edge and the height, not with the area: the small deck has a lot of perimeter for few square metres. On the 5 m² deck, railing and stairs are worth more than half the price; on the large one they disappear into the bill. It is the fixed cost disguised as an accessory.

When is the railing mandatory?

Above a height threshold from the ground set by local codes — typically between 50 and 100 cm depending on the jurisdiction. Below the threshold it is a choice; above, it is a legal item, with its own minimum heights and baluster spacing. That is why the level is decided before the quote.

What does the site surcharge include?

The drop that demands posts and sized beams, the ground to level or drain, the weed membrane, difficult access and obstacles to build around. It is the item that turns the deck into a stilt house: it is declared at the survey, walking the footprint.

Wood or composite (WPC)?

Real wood costs less upfront (pine) or competes with composite (exotics) but asks cyclic maintenance: oil or preservative every 2-3 years. Composite costs like a mid-high wood and nearly zeroes maintenance, but heats up in the sun and is not repaired: it is replaced. The mature comparison is on cost per year of life, maintenance included.

Is the substructure included in the price per metre?

In the serious package yes: joists, footings or adjustable pedestals, stainless fasteners and installation. The «boards only» catalogue price is typically half the installed one — and the substructure is the invisible half that decides how many years the visible one lasts.

Does the calculator work in square feet?

Yes, by converting: 1 m² = 10.8 square feet, so a price per square foot is multiplied by 10.8 to become per square metre. The index ι does not change with the unit, because it is a ratio: just use area and cost in the same family.

How much maintenance does a deck need?

It depends on the species: treated pine wants oil or preservative every 2-3 years, exotics age to grey without losing structure (the oil is aesthetic), composite just gets washed. Everywhere the ventilation under the boards and the cleaning of the gaps matter: that is where a deck rots, not at the surface.

Do you need a permit for a deck?

It depends on height and size: the ground-level deck is often permit-free, the raised or large one may require building paperwork, and local codes set the thresholds. Checking before the quote avoids the work to be regularised — which always costs more than the paperwork.

What is NOT included in the calculated cost?

Pergolas and roofs, integrated lighting, building permits where required and the garden works around. The model photographs the deck: floor, framing, site and perimeter — the rest is added item by item.

Updated July 2026