Please get your design and layout sorted first, using a Professional Design Team (Architect, Engineer). at this stage you should be informed of all constraints and options to maximise the quality of the space, and the ROI.
you are in control.
only then should you consider taking your project to builders. your design team should have good working relationships with excellent builders.
this allows you to be best in control, and have the time to really best plan all the nice touches than can make your basement streets ahead of others.
you are about to commit to a big spend, make sure you get the design that works best for you and the property.
choice of contract is paramount - make sure you don't simply agree to the T&C's of a builders contract. architects can advise on all this to help best ensure your project is delivered on time and to cost.
some may perceive that using a design team adds costs - but of course, basement companies have to pay for engineers, architectural designers etc.
we did something special for a client on mallinson road.
http://www.atelierdb.com/architect-lond ... andsworth/
Hope this helps.