Awesome city, it’s really a shame what’s happening in the EU now. I don’t want to side track the thread I’ll stop. 
As far as I know there is not a way to drag a box and just rebake the current selection.
What you can do (which is what I do in my projects, is divide your projects up in parts and move every part to a seperate layer.
If you make sure that you have your light setup in the top layer (which should always be on) you can then hide certain layers with geometry that you dont want to re-bake.
I usually put the foundation (walls, ceilings etc.) of my apartments on one layer, and make a seperate layer for each room that contains it’s furniture.
I know its a messy work around, and it doesnt always work in every situation, but it can be useful for some projects.
You can read more about this layer system here: World Composition User Guide | Unreal Engine Documentation
The way I do contracts, and it took sometime, is to bill hourly. When I give a quote, I give them a fee range. Not one hard number. And I work hourly to stay with in that range. I keep very detailed hours of every project. I go to meetings for new projects with images and invoices of past projects that are similar to theirs so they can see what is involved. And clients love it when I come in under the fee range. They start to earn your trust over time. My goal is to get paid for every minute I sit at my desk working. No more, no less.
How do you handle clients that say I want a flat fee, no exceptions?
I give them a quote that is the middle of my projected fee range. That will cover some changes upfront. After I pick up their comments and changes, I present them with a Proposed final image. And at the time, as stated in the contract, all changes after that are done hourly. I just make it clear. I have lost jobs due to my method. But I have gotten more than I have lost.
Makes sense to me.
Or charge a flat fee like MIR.no @ 4000 euros/render 
source : chaosgroup labs podcast 50
4000 Euros for which render? A high rise probably, a kitchen no way. If you’re doing high rise or large building renderings you have to have background photos which cost money/time, so 4K does not seem like a lot to me.
Only a few select artists/studio can do that. Some people want a MIR render and are wiling to pay, like a luxury product.
Check it out https://labs.chaosgroup.com/index.php/cg-garage-podcast/cg-garage-podcast-50-3d-symposium/. They talk about pricing, workflow, industry’s future, etc. This is from the panel in Cancun!
I listened to some of that thanks. I thought what he said about they don’t remember how much time you were given only what the quality is was spot on. I can’t tell you how many times I busted my *** staying up all night to do jobs only to have someone say 6 months later, that wasn’t your best work. LOL!
I was gonna say 4k is pretty fair for an exterior of the calibre they provide. I thought they would charge more since the renderings also double as unique art pieces.
VXGI ArchiViz could potentially solve many of the issues you raise, if it can scale up accordingly in the next several years.
Besides that, Octane GPU rendering and similar in UE4 could also take baked lightmapping to the next level.
hey I’m dealing with same limitations. most archviz projects (non minimal) are big, lots of decorative and furniture. moving objects around is part of what we do to achieve best scene composition. for bunch of us that are comfortable with 3dsmax and vray presentations, unreal realtime presentation is too early I think; all because of insanely long time lightmass build time and issues you’ve posted here.