Since you said, this is a "massive renovation" on your home. I assume, you have an Architect?????
If you have an Architect... they typically have, their own team of Contractors/a company they use/subcontractors etc.
AND they... get the Permitting, for you.
That is their, job.
AND the blueprints of course.
As Jim At Home Dad said... "overages" HAS TO BE factored into a bid.
Some bid low. BUT.. that means, any overages, will cost you. More. And they know this. Versus a GC that will maybe bid higher, BUT they factor in, appropriately, any overages. So, a bid is not just a bid... based on who is lower than the other.
Bear in mind, that overages, costs... overall, more on a project. AND it cannot be avoided totally... because on any job/construction, there will be overages as the job goes along and any unexpected situations that comes up... as they are demo'ing your home etc. Or the existing structure, has inherent problems with it, which they need to correct etc.
With any massive renovation... an Architect is needed. Because, they know structurally, and design wise, what you want and they do the blueprints/elevations/electrical/lighting/plumbing specifications per the plans/designs.
"Specifications" on a job/job site... is important and a layperson cannot do this succinctly. You would not know, from the ground up, what is needed on a job site per the renovations, unless you also have, construction background.
AND or, you may need an Interior Designer... unless the Architectural firm has one as well. Some Architects do interior design as well or have an in-house, interior designer. This is different, from an Interior 'decorator.'
EACH trade, has a specific, purpose. And knowledge.
Contractors/sub-sontractors, will, do installation and fabrication, or their Sub's will.
And as Jim At Home Dad said... SIGNED contracts.
Make sure all involved/the company you use, is insured and bonded.
Get references as well.
EACH trade as I said, has a specific purpose.
There is the GC, who is overseeing everything. And they coordinate the Subs. And is your point person. They are like a Project Manager. He/She then has a team that does the work. Sub-Contractors, are anyone in the trades, such as Electrical or Plumbing, Installers, Fabricators, concrete, Excavators, etc. that are needed on the job to complete it. They have to be, licensed ALL of them.