Yes 1-4 will be cheaper prior to construction. But at that point you are skipping drywall, trim, floors, and AC... 10k? Would it be better to finance that in the mortgage versus doing it later? (It’s easy to spend someone else’s money)

@HALLETT & Co. Since an attached garage is typically included in the cost/SF of a house, what fraction of that would it be to finish a bonus room involving steps 1-7 above?

E.g. Construction cost is $200/SF, would upgrades and finishing bonus room be like $40/SF?

How many square feet is the house without the 800 sq foot bonus room? Are you doing this in two steps in order to keep the cost down during the initial construction?

Installing the a mini split after the house is built will cost $5-$7K in most parts of the country. Whereas using the main HVAC systems is should be less than $1K, or possibly more if you made it a separate zone. Unless you plan to do the work yourself I think you are better off completing the bonus room during the initial construction.

1-4 will definitely be cheaper to do during construction. Talk to your architect or engineer early on. Depending your requirements the roof and floor system can get quite complicated.

Regarding HVAC, I agree with @mike_home. We have a similar area in a new build and using the main HVAC system on a separate zone added about $1,500 to the total HVAC. Using a mini split was more money and required a backup heat source because we are in a pretty cold climate.

There may be specific code requirements to put living space (especially a bedroom) above a garage, so check with your local code enforcer. If there are specific requirements, it surely is easier to include them from the start

@emilyam819 : It has two 36" x 42" windows. I have updated my original post with an image of the proposed bonus room. I plan to use it as general use, open space => exercise room etc.

I am building outside city limits and so no code restriction.

@mike_home : The house alone is 3800 SF (then the bonus room if finished will add 800 SF to that. There is also the opportunity to finish the basement and get 1500 SF more!).

Yes, i was thinking of keeping costs down because the builder charges the same rate for bonus room as other rooms (cost/SF) even though most structural components of the bonus room are a part of the garage and that is already calculated in the cost/SF of the home.

I was thinking mini splits are more efficient but good to know that it is possible to expand the second floor's HVAC and that would be cheaper and condition the room well.

@jackson28 : Would the recommended floor system be I-Joists? Thanks for the tip on utilizing the home's HVAC.

@Jon John I-Joists may be required but that is really a question for your architect/engineer.

We recently built a house with approx 600 sq ft of living space about the garage and did not use I-Joists. However, the trusses were expensive and complex. Floor framing and bracing was extensive. I-Joists might have been easier from a labor standpoint but more expensive from a materials standpoint.

We did what you are contemplating but thought we would save money and tax Bill. It was less expensive and we designed HVAC to be easy add later. I put loads of electricals so we had flexibility down the road. That was less cost to do at construction. Certain fire code items were needed as well. Truthfully, that was about 2/3 of the cost. So, we saved but then when we finished off that was cash out of pocket. So it’s a choice. If I had it to do again, I would do it all at initial build. Ours was 750 sq ft heated/cool space. And I wouldn’t have had the mess after moving in. Getting sheetrock up to second floor was messy! Good luck.

@jackson28 : Is the truss you mentioned a roof truss with an integrated living space built into its design? Like in this link

Yes and no. I have a few attic trusses but mostly scissor trusses. Both sides of my garage have large dormers to maximize space. My truss design is ridiculously complex - my biggest regret with this build - so I can’t really recommend this approach.

IMO have specific reason for the space and get it done now , the best time and also probably the best money saver too. IMO a bonus room is useless with a use for it when planned.