3D Wooden Building designer

We sell highly customizable wooden sheds. We need a way for the customer to alter the size, layout, windows, doors, roofing. And add on extras such as window boxes, cat flaps, extra height, partitions etc. These all have rules which we will work with you to set up in a database or Object Class.
The Designer needs to work with desktop browsers, mobile & tablet browsers. So perhaps HTML5 would be a good way to go with this as opposed to Flash.

We wouldn't want to require a user to install a plugin to use it.
We would want to give the user a price as they are designing.
To be able to save their designs.
To be able to share their designs.
To show the dimensions (in as much detail as possible) on the design.
To allow the user to switch between imperial & metric measurements.
For the image to change colour based on the type of wood chosen.
To be able to see inside the shed for placement of shelving, partitions etc.
We would need a way to add more availble layouts, extras, etc.
We would like the source code.
The code would have to work within a .Net C# MVC MySql project.

Examples (although more complex) would be http://www.homestyler.com/designer, http://www.easyplanner3d.com/interior_design_software/3D_design.php

Our site is http://www.easyshed.co.uk
The current shed builder step by step process is here:
http://www.easyshed.co.uk/shed-builder
And the current shed builder is here:
http://www.easyshed.co.uk/shop/custom-sheds

So we have created a few hundred static images to cover a few variations, but there are thousands and so we need an automated way to generate a picture of the user's shed.