The short answer
A 20 square metre deck in the UK typically costs around £2,000 to £5,000 supplied and fitted, depending mainly on the material. At roughly £100 to £150 per m2, a softwood 20 m2 deck often lands near £2,000 to £3,000; at £150 to £250 per m2, a hardwood or composite deck of the same size sits closer to £3,000 to £5,000. The total covers the groundworks, subframe, boards, fixings, finishing and labour. A raised 20 m2 deck with steps and a balustrade costs more again. The per-metre rate often eases a little at this size compared with a tiny deck, because set-up costs spread further. Always get an itemised quote for your own garden.
Twenty square metres is a common, usable deck size — roughly four metres by five — so it is a practical anchor for budgeting. Scaling the per-metre rate to this size shows how material choice swings the total.
20 m2 deck cost at a glance
- Typical total rangeAround £2,000–£5,000
- Softwood 20 m2Roughly £2,000–£3,000
- Composite / hardwood 20 m2Roughly £3,000–£5,000
- Raised 20 m2 deckHigher again, with steps and guarding
- Rough dimensionsAbout 4m x 5m
What a 20 m2 deck costs by material
The clearest way to budget for a 20 m2 deck is to take the per-metre rate for your chosen material and multiply it by the area, then treat the result as a guide rather than a fixed price. The figures below are indicative supply-and-fit totals for a straightforward ground-level 20 m2 deck, for guidance only.
| Material | Indicative per m2 | Indicative 20 m2 total |
|---|---|---|
| Pressure-treated softwood | Around £100–£150 | Around £2,000–£3,000 |
| Hardwood | Around £150–£250 | Around £3,000–£5,000 |
| Composite | Around £150–£250 | Around £3,000–£5,000 |
Indicative UK figures for guidance only; a raised or complex 20 m2 deck costs more. Sources: Checkatrade and MyJobQuote 2026 decking cost guides.
Why the rate eases a little at this size
A 20 m2 deck is large enough that the per-metre rate often comes in a touch lower than for a very small deck. This is because a deck has fixed costs that do not scale with area, and at 20 m2 they spread over more metres:
- Set-up and access: bringing tools and materials to site, and clearing up after, costs much the same whether the deck is 5 m2 or 20 m2.
- Foundations and edge detailing: the perimeter and corners carry a disproportionate share of the work, so a larger deck dilutes that across more area.
- Setting out: planning and marking the deck is a fixed task that a bigger area absorbs more efficiently.
So the same fitter may quote a slightly lower per-metre rate for 20 m2 than for a 5 m2 deck. The effect is modest, not dramatic, and it reverses if the larger deck is more complex — a 20 m2 deck with curves, multiple levels or a raised section can cost more per metre than a small plain one. As a rule, area helps the rate while complexity hurts it.
What changes the total for a 20 m2 deck
Even at a fixed 20 m2, the total can vary widely, because area is only one of several factors. When budgeting and comparing quotes for a 20 m2 deck, the things that move the figure most are:
- Material: softwood, hardwood or composite, which is the single biggest swing as the table shows.
- Height: a ground-level 20 m2 deck is the lowest-cost; raising it adds foundations, posts, steps and a balustrade, and can bring planning and building regulations into play.
- Ground and access: a flat, firm, accessible garden is quick to work; a sloping, soft or enclosed one takes more groundworks and labour.
- Shape and detail: a plain 4m by 5m rectangle is the most economical; borders, curves and multiple levels add cutting and time.
- Extras: steps, balustrade, fascia, skirting and lighting are often priced separately and can add a meaningful amount.
The honest position is that a 20 m2 figure is a starting point, not a quote. Fixing the material and confirming the height and extras, then asking fitters to itemise the groundworks, subframe, boards and labour, turns the rough range above into a reliable budget for your own deck.
Frequently asked questions
How big is a 20 square metre deck?
Twenty square metres is roughly four metres by five, though any combination multiplying to 20 works — for example two by ten, or a stepped shape covering the same area. It is a common, usable size that comfortably fits a table, chairs and some planters, which is why it is a popular anchor for budgeting.
Is a 20 m2 deck cheaper per metre than a small one?
Often slightly, yes. Fixed costs such as set-up, foundations and setting out spread over more area on a 20 m2 deck than on a tiny one, easing the per-metre rate a little. The saving is modest, and it reverses if the larger deck is raised or has curves and multiple levels that add complexity.
Does a 20 m2 deck need planning permission?
Area alone does not usually trigger planning, but height and garden coverage can. A deck more than 30cm above ground, or one that takes the total of decking and extensions over half the garden, may need planning permission. Conservation areas and listed buildings have stricter rules, so check with your local planning authority.
Sources & further reading
Figures on this page are typical UK ranges drawn from published sources and depend on your specific garden. They are guidance, not a quotation.