Extensions next to trees

The timber frame will need a foundation to sit on, the area will be prone to movement so no way of building on it

Perhaps a raft foundation is what you want, check with Building Control to see if they are ok with it.

Not easy to give a short answer, so get the kettle on... There are too many variables to consider without being able to see it in person, so the easiest thing to consider is bridging the tree root area. Depending on the length of bridging required, it can be done with timber, and assuming that the extension is only going to be one storey then it's easy enough. Based on a one storey extension of timber frame, you will need some kind of support about every 1m to 1.2m. You can use a dense concrete block levelled onto the ground by taking out enough soil to make the block as level as possible and then put a post 3" to 4" square on it, then these: https://www.ebay.co.uk/itm/363741814019?hash=item54b0b39d03:g:-f4AAOSwn1liF5-9 then the joists fixed within those supports. ALL supports must be within the 1m to 1.2m distance and then use any spare timber to brace the uprights to the joists. If you want, you can get some 3" x 1" timber and use them at 45 degrees as bracings. Use them liberally and in every direction. You can also use these: https://www.screwfix.com/p/simpson-strong-tie-light-restraint-strap-1000mm-flat-10-pack/68346 The joists will need to be spaced every 400mm or less. If you find that whatever flooring you want to use is going to miss a joist, then add another, don't try to bridge that bit. If you are going to put anything heavy in the extension, like a hot tub, then double up on the amount of framing mentioned! ONLY use treated timber. Bolt a wall plate (The wood that you fix to the house wall, which should be 4" x 2" and full height for the extension.) to the house using masonry bolts and something like anchor set which you can find on Screwfix, etc or local builder's merchants. Bolt every 300mm as a minimum. Do the same for the horizontals. Use expanding foam behind these bits of timber to make a good weather seal, just let it go off and cut it back the next day and then use a nice sealant to make it look nice. Build your timber framing with 4" x 2" with 400mm centres and then add in bracing for where any windows or doors are going. Add weather proofing membrane to the outside of the wall frames, add whatever cladding you want and then stand them up. Then screw them to the wall plates every 6" to 8", staggering them an inch either side of the centre line. If necessary, use some scrap timber to put bracing up to hold the wall up whilst you add another side to keep it upright. Use feather edge to enclose the gaps under the joists to the ground and keep an eye on them... If the ground is shifted by the tree roots, the tree won't care about the weight of the extension, so set the supports as long as you can because if the ground rises, the supports can be shortened to take up the space. If the worst should happen, and the tree decides to move the extension, the timber uprights on the blocks can be removed one at a time and be moved/shortened/replaced/other. But keeping an eye on the feather edge at the bottom will show if there is any ground movement for as little cost as possible and you go under and move the uprights and supports. If you are planning something more technical than this, find a structural engineer and ask them to come and have a look. There is always a way, it just depends on the cost! The easiest solution would be to remove the trees, but if they are not yours or they have a tree preservation order on them, you are a bit stuck, so bridge over them. Your extension can always be taken up a step or two if you need to gain height to put blocks and supports under it. Lastly, you will need an inspection from building control if you want to knock any wall out to make room for windows or doors if you don't already have a door where you want it and the floor of the extension cannot be more than a foot above the highest bit of land on your property. So, if either are a problem, get hold of your local planning and inspections folks first! They will usually help you for free or very little at the beginning. Though some authorities do charge.

You my be able to build the extension using piled foundations and ring beam.have a look on the internet for pilling companies,and have a look at what is possible,hope this helps regards Simon.