What Changes a Garden Room Quote: 8 Things That Move the Price

Ask three London installers to price the same 12 square metre garden office and you can easily get three figures that sit five to fifteen thousand pounds apart. That spread unsettles most homeowners, because on the surface the buildings look identical. They rarely are. A garden room price is the sum of a series of choices about what goes into the structure and how it reaches the bottom of your garden, and once you know which choices move the number, a confusing set of quotes becomes easy to compare. Absolute figures vary by year and specification, so treat every price here as an indicative range rather than a fixed rate, and the full tier-by-tier London price breakdown sets the baseline these variables move around.

These are the eight variables that do most of the work, in roughly the order they hit the bill.

1. Foundations and what is under your garden

Foundations are the first place two quotes diverge, because the right answer depends on your specific plot rather than a catalogue. Much of London sits on clay that shrinks and swells with the seasons, and gardens are frequently criss-crossed by protected tree roots, so the choice between a poured concrete slab and driven ground screws is site-led. Ground screws typically run from around £1,500 to £3,000 for a standard room, while a concrete base often lands a little higher and takes longer to cure. A sloping garden, poor access for a mini-digger, or old concrete that has to be broken out and carted away can all push this line well beyond the baseline. The trade-offs between driven ground screws and a concrete slab turn on exactly these ground conditions, and Checkatrade's ground screw installation costs sit in the same bracket as an independent check.

2. The shell: SIPs versus timber frame

The structural system is the single biggest cost driver after size. A traditional insulated timber frame is the cheaper starting point. Structural insulated panels, or SIPs, cost more upfront but give a warmer, more airtight shell with fewer cold bridges, which is what makes a room genuinely usable through a London January. A quote built around SIPs will read higher than one built around timber frame for the same footprint, and that is a difference in specification, not a difference in value. When you compare quotes, the first question is always which system each one is pricing, because otherwise you are comparing two different buildings.

3. Insulation and the U-values behind the wall

Two rooms can both be described as insulated and perform completely differently. The depth and grade of insulation in the floor, walls and roof determines whether the space holds heat or leaks it, and that shows up in the U-value each installer will quote if you ask. A budget build might use 50 to 70 millimetres of PIR board; a year-round specification uses considerably more, plus better airtightness detailing. Thicker insulation and proper detailing add cost, but they also cut the heating bill for the life of the building, so a slightly higher quote here often works out cheaper to run. A room specified for genuine year-round use is built to this standard as a matter of course rather than as an upsell.

4. Glazing, doors and acoustic glass

Glazing is where personal taste can add several thousand pounds. Standard double-glazed windows and a single door sit at the bottom of the range. A three-panel set of aluminium bifold doors, floor-to-ceiling glazing, or a lantern rooflight each carry a premium, and the frames matter as much as the glass. Homes near flight paths, railway lines or busy roads often need acoustic glass, which typically adds a fifteen to twenty per cent premium over standard units. None of this is waste. It is simply a set of choices that a quote should itemise so you can see what you are paying for rather than reading one lump sum.

5. Size, and the thresholds that come with it

Price scales with floor area, but not in a straight line, because size also trips regulatory thresholds that carry their own cost. A garden room usually stays within permitted development when it is single storey, no taller than 2.5 metres within two metres of a boundary, and covers no more than half the garden, and the official Planning Portal guidance on outbuildings sets out the full conditions. Push past 30 square metres of internal floor area, or add sleeping accommodation, and building regulations approval is triggered, which brings inspections and certification costs that a smaller room avoids entirely. So a jump from 15 to 32 square metres does not just add build cost, it can add a whole compliance layer, and a good quote will flag that rather than let you discover it later.

6. London access, cranes and getting materials in

This is the line that catches people out most, and it is almost entirely a London problem. If materials cannot pass through a side return, they either go through the house with all the protection and extra labour that involves, or they are craned over the roof. A crane day in the capital is a meaningful cost on its own, and parking suspensions, skip permits and restricted delivery windows stack on top. Two identical builds on two different streets can diverge by thousands purely because one has a wide side gate and the other has a terraced house with no rear access. Any installer who has not asked how they will physically get to your garden has not finished pricing the job.

7. Electrics, plumbing and climate control

A bare shell and a fully serviced room are very different quotes. Running a dedicated armoured cable back to the house consumer unit, fitting lighting and sockets, and certifying the work under Part P is a standard cost, and a long buried cable run adds to it. Add a toilet or a small kitchen and you are into drainage, which is the most expensive single upgrade because it often needs a pumped connection where the garden sits below the house. Air conditioning and underfloor heating each add a further layer. Whether these are in the price or listed as extras is one of the most common reasons two quotes look far apart when they are actually pricing different levels of fit-out.

8. Internal finish and certification

The last variable is how finished the room is when the installer hands over the key. A budget quote might leave you with exposed timber or unpainted board and a basic electrical package. A higher quote includes plastered and painted walls, quality flooring, joinery, and a full set of certificates covering the electrics and, where relevant, building regulations. That paperwork is not bureaucratic padding. When you come to sell, a buyer's surveyor will ask for it, and its absence can stall a sale or knock money off the price, which matters because a well-built room is one of the features London buyers actively look for.

How to compare quotes like for like

Once you can see these eight variables, the trick is to make every installer price the same brief. Give each of them the same size, the same foundation type, the same glazing, the same standard of finish and the same access constraints, and ask for the VAT-inclusive figure, since the twenty per cent almost always applies unless the room is used exclusively for a VAT-registered business. A quote that is dramatically cheaper than the others is usually cheaper for a reason, and that reason is normally hiding in one of the eight lines above.

It is also worth holding a garden room quote against the alternatives before you commit. For a like-for-like footprint it typically comes in well below the cost of a rear or side house extension, and if the total is more than you want to fund in one go, there are several ways to spread the cost of a garden room that change the monthly picture without changing the spec.

The bottom line

A garden room quote is not a single price, it is eight decisions bundled into one number. When you understand what each of them does, a scatter of quotes stops looking arbitrary and starts telling you which installer has priced the building you actually want. If you would like quotes you can compare with confidence, share your plot size, intended use and access on the form on this page and we will match you with vetted London specialists who price to the same clear brief.