I am in the planning/ cost estimation for decking in our back garden. As the title says the first question is can I use non external coach bolts? Is it best to use external ones? I have never noticed any in use that are external coach bolts, they always seem to be the standard steel ones. A bit of info. I fist looked a a retaining wall but the garden in nearly 9 meters wide. the foundation and concrete blocks were near the £1000 mark without mortar mix or the surface material. So I am now looking at a deck. At the moment the grass is steep and uneven, so I plan on removing some of the soil to lower it to a more even slope. I was going to attach the deck to the concrete, however when I looked at it the concrete is not very thick and is resting on soil I think. I will dig down and investigate when closer to forming the foundations. A few additional questions. Will I need noggins or will 600mm space be ok without them? There is a public sewer running down the middle (red yellow box is access pannel) so will not be able to support it in the middle. I read (and can no longer find where) I read it to have a double frame. Is the best option? Does it make the frame stronger? Your opinion and results for post in concrete or anchor in concrete post on anchor? I have read different articles recommending different approach. Every time I do I keep thinking that my plan is wrong. Thank you for reading so far and I appreciate and information you can give, weather you answer a question or simply give your advise. I have added some images from Sketchup, I have not been able to attach the Sketchup file.