Garden Room Calculator

Materials for a self-built timber frame garden room run from roughly £5,900 for a 3 x 2.5m up to about £17,000 for an 8 x 4m, based on live prices from UK merchants. That covers everything from the concrete base to the bifold doors, at first fix. It excludes labour entirely, because this is a DIY build. It also excludes electrics, plumbing and internal fit-out, which are shown separately as placeholder costs. Buying a comparable garden room supplied and installed usually costs two to three times the materials figure.

Usually not. A garden room normally falls under permitted development if it is single storey, sits behind the front elevation of the house, and covers no more than half the garden. Two things commonly break that. Any part within 2 metres of a boundary must be no more than 2.5 metres high overall, and an internal floor area over 30 square metres needs full planning permission regardless. Listed buildings, conservation areas and Article 4 areas have their own rules. The calculator flags the thresholds as you change the size.

Only with a very low wall. The 2.5 metre limit applies to the highest point of the finished building, measured from ground level, and it is easy to underestimate what the roof adds. A 2.1 metre wall plus a warm deck on 150 to 200mm rafters comes to roughly 2.62 to 2.70 metres overall once the rafters, firrings, deck, insulation and membrane are counted. That is over the limit. To genuinely clear 2.5 metres you need to lose height somewhere: a shallower floor build-up, thinner roof insulation, or a wall around 1.9 metres. Our calculator counts the whole build-up and tells you the real number rather than the flattering one.

Under 15 square metres internal floor area, an outbuilding with no sleeping accommodation is generally exempt from building regulations. Between 15 and 30 square metres it is exempt only if it sits at least 1 metre from any boundary, or is built substantially of non-combustible materials. Over 30 square metres it needs building control regardless. Electrical work is notifiable under Part P whatever the size, so the final connection needs a registered electrician.

Pine shiplap is the cheapest to buy and the dearest to own, because it needs re-treating every two to three years. Western Red Cedar costs more up front, silvers naturally, and needs stainless fixings because it corrodes ordinary ones. Composite is dearest to buy and needs no treatment at all. Over a ten year period the three end up much closer together than the sticker price suggests. The calculator prices all three so you can see the up-front difference on your own size.

You need a beam, but not a steel lintel. A pressed steel lintel is a masonry component: it bears on blockwork and carries brick, neither of which exists in a timber wall. The standard detail is two C24 timbers on edge, bolted together with a packer, sized on deflection rather than strength. A 3 metre opening in a 3 metre deep room needs about 200mm of depth; a 4 metre opening in a 4 metre deep room needs about 230mm. Bifold manufacturers often specify a maximum 3mm deflection over the head, which no stock timber section meets at 3.6 metres or wider, so those openings need a flitch beam or a steel designed by an engineer.

A reinforced concrete slab is the usual choice and the one this calculator prices: sub-base, sand blinding, damp proof membrane, A142 mesh and 150mm of concrete, with the timber frame sitting on engineering brick courses above ground level. Ground screws and a pad-and-bearer base are cheaper and quicker on a sloping or root-filled site, but they need a supplier survey rather than a takeoff, so they are not priced here. Whatever the base, the frame must be isolated from ground moisture by a DPC.

Two competent people working weekends typically take six to ten weekends to reach first fix on a mid-size room, plus curing time for the slab. The base is the slowest and least forgiving stage. The frame goes up quickly. The roof and the cladding take longer than people expect, and the bifold doors usually have a lead time of four to eight weeks, so order them early rather than at the point you need them.

Because this is a tool for people building it themselves. Adding a labour figure would invent a number that does not apply to you, and it would hide the thing you actually came for, which is what the materials cost. If you are getting quotes instead, the materials figure is still useful: it tells you what your builder is paying, so you can see the margin on the supply side of the quote.

Live prices from UK builders merchants, the same data behind the rest of PriceNailer. Every line in the materials list shows whether it is priced live or estimated, and how many merchants stock it. Specialist items like EPDM systems and aluminium bifolds are single source and are labelled as such, because a cheapest-of-14-merchants figure on a product only one merchant sells would be a fiction.

No, and that is deliberate. The engine computes the exact requirement, then rounds up to whole purchasable units and reports both numbers so you can see what the pack sizes force on you. Timber is nested against real stock lengths rather than padded by a percentage. The usual trade rule of adding 10 percent actually under-buys timber on every size in this calculator, because it ignores how the offcuts fall.

Yes, but it changes the job. A garden room WC usually needs either a gravity connection to an existing drain, a small-bore macerator, or a packaged pumping station, depending on the distance and the level difference to the nearest manhole. Connecting to a public sewer may need a build-over agreement from your water authority. Adding a bathroom also tends to push the internal area past 15 square metres once the internal walls go in, which brings building control into play. This calculator stops at first fix and does not price the fit-out.