We’re planning a 3m x 3m corner room for the garden from Dunster House or similar, but will need to clear and level some lawn to do so.

Do I need a hardcore and concrete base for it? Or is there any other options please guys? And are Dunster the best for garden/summer rooms?

Not for a garden room but for a shed I just removed the turf, put down weed suppressant and supported a wooden raft on breeze blocks.

It might be worth asking on the D3/D4 forum as I've seen this come up a few times and there are some very knowledgeable folks on there. I recall one of them recommending some ground screws which can be screwed into the soil to form a quick, level base for buildings. 2016 Skoda Octavia VRS Estate
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It depends what you are putting on it. If it’s just a small shed based sour side room then a base of hardcore, the 2’ x 2’ x 2” slabs bedded on a sharp mix will do fine.

If it’s larger then strip the turf and topsoil, base in some hardcore/type 1 Mot, then form a perimeter for a concrete slab. This can be reinforced if the room is really big, but normally concrete on a well compacted base will be plenty. Any of the suppliers should be able to advise on their particular model.

Never had a dunster house shed, but there are plenty of companies out there. The specs to look at are:

Type of wall cladding and thickness the thicker the better. Tongue and groove best, ship lap is good overlap feather edge not so much.
Framing size 2x2 3x2 or 4x2
Floor material and thickness. OSB, Ply, etc the thicker the better. 18mm is ideal
Roof structure material as for the floor
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Couple of threads related from D3/D4

https://www.disco3.co.uk/forum/concrete-ba...75705.html

https://www.disco3.co.uk/forum/summerhouse-build-156754.html

The first mentions the ground screws. 2016 Skoda Octavia VRS Estate
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Depending on the quality and compaction of the soil it will be sitting on, dig down approx 600 deep x 450 square. Blind the bottom with about 150mm of 19mm blinding stone graded to dust. Tamp down and sprinkle with a touch of cement for a dry mix to bind. Sit Concrete building blocks 7.3N 440mmx215mm on the blinding and level. Space at 1.5m centres. Site your vertical wooden prop on that to secure the base ,backfill and away you go. Don't forget to treat the timber. (Blackjack )  Tony
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Given its size and intended use I can't see why hardcore, sharp sand and concrete paving slabs won't do the job. Current: FF Westminster money pit

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Don’t have enough information to form an opinion and comment.

Is the soil dry all year or become water logged
Is the site sloped
Clay, sandy, gravelly or rocky soil

It’s a pain to set the ‘shed’ and then for it to move due to soil conditions or other issue like water problems

Cheers guys, will look into it more once it brightens up a little. Only downside of having the Mrs at home all day every damn day... keeps throwing stupid spur-of-the-moment ideas at me