Shed / Workshop Extension Proposal.

[Prototyper]

I used to have a nice size workshop for my DIY & engineering hobby interests. I built that one with the help of my late father, some 25 years ago. It’s still there on Google maps! It was 5M square & 3.5M high.

I’ve now moved house, and I need another place to work.
My present house has a small brick built shed at the bottom of the garden. It is approx 4m wide, but only 1.3m deep, so there is insufficient room for me to even stand at my lathe!

I propose to extend the existing brick shed out from the front.
My neighbours do not object, as they have done similar things. Although this new project will not be as big as my old workshop, due to available space, so I want to make the best use of what I have.

As I’m older now, and not able to build another workshop by myself, and do not have anyone available with building experience to help me, I’m trying to find a competent builder who wont rip me off. I would be grateful for anyone to review & comment on my ideas for a new workshop:-

The overall size of the extension (Outside dimension) is to be approx 3.8M square X 2.3M high.

It is proposed to be attached to the original structure using the Furfix starter system.

The construction will be 2 courses of engineering bricks up to DPC, then the walls in 100mm (I hope?) high density block.

I’m hoping there will be no need for brick piers with 100mm blocks on a 3.8m run, so as to create a flat, uninterrupted interior for better use of the space.

I would like to avoid having to use wider blocks if possible, i.e. 140 or 200mm, as it would make the walls heavy and would reduce the interior space, as well as needing a thicker base and more cost of concrete, but all constructive comments on this will be welcome.

There will be the need for groundwork to be done also, such as the concrete base and footings. The thickness of the base need only be minimal (150mm?) as it will never have a car on it – just a couple of machines - a small lathe, table saw, drill press, benches etc.
I would like comments as to whether I would need steel reinforcement and a DPM in a floor this thin?

The roof will be a sloped flat construction, (this would tie in easier with the existing shed’s flat roof), timber rafters (150mm X 47mm C16 treated?), sheathed in 18mm? WPB and covered with mineral felt.

There will be an approx 1.5m wide window in the front and a 900mm wide door in the side.

The walls will then be painted with an exterior protective coat, such as Dulux Weathershield or Sandtex. This lasted 15 years on my previous workshop at my old house, before needing re-painting.

This is not that much of a job for a good builder, as all of the finishing off, I would be able to do myself. There will be no insulation, plastering or painting etc needed.

Any constructive comments & additions will be welcome. And if any builders can estimate the current cost of a project like this, that too would be good – just so I do not get “stung” like I have in the past!

Thanks to anyone who takes the time to read this post.