I’ve been paying more attention to airspaces now and have discovered how boxed in my local airport is. There is class C space at 1600 feet over the whole field with a pattern altitude that is roughly 1020 feet. Some of the downwind legs are also tight for trying to do a 45 entry.
It looks like it would be really easy to make an incursion.
Being at 1000 VFR with Class C starting at 1600 is plenty of room.
If you do need to go through it then Class C doesn’t require clearance only to be in radio contact with ATC. Controllers will already be aware of class C near airports and you shouldn’t get many problems.
Class B is more restrictive and often around larger & busier airports but that’s not to say that you still won’t get permission to transition.
I figured radio contact would be okay. Although that adds some complexity with needing to be on two frequencies in an ofted congested traffic pattern. In MSFS, if I have copilot radio on and try to fly the pattern it will switch over to ATC.
My instructor seemed very concerned about keeping clear of that class C airspace.
The vast majority of GA traffic on FR24 seems to avoid flying through any of it.
Maybe it’s country specific (doubt it), but where I fly (IRL) class C definitely requires clearance. You’re not allowed to enter controlled airspace without a clearance.
There is no need to use two radios. Actually, none of the GA airplanes I have flown even had 2 radios. In general it works like this: a few minutes before the class C you let whoever you’re on with (like FIS if you’re in G) know you’ll be switching. You contact the class C ATC (likely a tower or approach), tell them what you want and ask for permission. You can enter when you get the permission. Until you do you can’t.
Answering the question you can get as close to it as you want. As long as you don’t fly into it without permission. Of course it depends on the context. When it’s tight you might be very close (at my home airfield on approach you’re practically scraping it). If you’re flying in G and closing in on a C you’ll keep a few nm distance before getting clearance, just to be on the safe side.
Also note that (at least in my country) class “C” around airports have precisely indicated entry points (and routes inside the C) for VFR traffic. You don’t just enter anywhere you want.
So if you start in an uncontrolled airfield under a “C” you don’t just climb and enter it. You need to go to the entry point, which will be outside of the uncontrolled traffic pattern area most likelly.
USA :
What must you hear to enter Class C airspace?
Note: When contacting ATC, "Contact with the ATC facility " you must hear ATC repeat your callsign back to you to have met the requirement of 2 way communication with you.
Just looking at it from a student pilot perspective, the idea of overflying the pattern altitude with 80 feet to spare seemed a little too razor thin.
I’ve simmed it a few times now and (depending on runway) I can about make it to 700 feet after takeoff before needing to make a crosswind turn before hitting the class C boundary. When I flew it for real it looks like I was much tighter and turned crosswind at 500-600 feet. The general things I’ve read usually seem to indicate keeping runway heading up to 1000 feet.
The students at another local towered airport seem to have a much easier workload! They just go where the tower says. 
You should turn crosswind when the aircraft is **approximately 300 feet below pattern altitude -
unless locally published procedures specify differently.
At least in the FAA world all you have to do is be in two way radio com with ATC controlling the Class-C. Means you call them up and as soon as they read back your call sign in any form you are good to go.
That means … anything from an eloquent „good day 8087, are you up for a GCA approach.?“ to „8087B roger“ is a green light.
On very rare occasions you may get „aircraft calling stand by“ … and that is a red light, because ATC has not explicitly used your call sign. So you do NOT have permission to enter yet.
If this is a training airport the nearby ATC controllers are used to lots of targets near their airspace. And often there are procedures in place, such as a quick call to the C-controller before taking off, to simply state intentions.
Some of that you can even find out by calling the facility on the phone and often they are more than happy to chat with you and give you helpful advice that, hopefully works for both sides.
Apparently it’s different under FAA and EASA.
Under EASA according to SERA.6001 Classification of airspaces
COMMISSION IMPLEMENTING REGULATION (EU) 2016/ 1185 - of 20 July 2016 - amending Implementing Regulation (EU) No 923 / 2012 as regards the update and completion of the common rules of the air and operational provisions regarding services and procedures in air navigation (SERA Part C) and repealing Regulation (EC) No 730 / 2006 (europa.eu)
Class C. IFR and VFR flights are permitted. All flights are provided with air traffic control service and IFR flights are separated from other IFR flights and from VFR flights. VFR flights are separated from IFR flights and receive traffic information in respect of other VFR flights and traffic avoidance advice on request. Continuous air-ground voice communications are required for all flights. For VFR flights a speed limitation of 250 kts indicated airspeed (IAS) applies below 3 050 m (10 000 ft) AMSL, except where approved by the competent authority for aircraft types, which for technical or safety reasons, cannot maintain this speed. All flights shall be subject to ATC clearance.
It’s funny how such a fundamental thing can be so different. I would assume they would have coordinated this.
I should state my location when asking such questions! It is the US.
This spot of airspace between the two airports is mostly empty from my observations. The only time I see traffic for the class C airspace on that side is during unusual weather events, such as bad storms to the opposite side of the airport.
I’m sure the controllers see way more training GA traffic on their border than they do entering the airspace. I’d be curious to know the level of accidental incursions.
From the looks of it, I’d guess most VRF privet pilots would just rather fly under and around than get clearance.
It’s not “clearance”, per se, but they still have the ability to deny you into the class C.
In the VFR world in the US, you don’t need to hear the word “cleared through the class C”. You just need for them to state your callsign. But if ATC were to say, “Aircraft calling in, stay clear of the C airspace”, without your callsign, you can’t get in. So it’s not explicit clearance, but ATC still has the ability to box you out.
So it’s more passive permission and an active rejection?
This specific class C is rather dull for the most part. I gave up monitoring it. The Class D airspaces and untowered airports seem to be livelier.
So if I wanted to overfly, I could contact the ATC, do a teardrop,then switch over to the airport’s CTAF?
Sorry if these are overly basic questions. I tend to learn by reverse engineering.
I was about to type the exact same answer but saw your post just in time. Here in NL C and D are controlled airspaces. I need to get clearance for everything: startup, taxi, take-off, leaving, entering, landing, and again taxi. On some airports even switch-off.
E and G s what we call uncontrolled. There it is recommended to advise one’s intentions on the radio.
I’m not a CFI, so anything your CFI, DPE, or flight school says should supercede me.
I think you could look at it that way, but the way I look at it is that you still need permission. It just comes in the form of two-way radio contact rather than the word “cleared”.
If you wanted to overfly with a teardrop into the downwind, I would definitely recommend talking to the class C controllers. While technically you could do it without talking to them, you would be within 100 ft of the shelf with that overfly and that’s way too close for my comfort.
Psychologically/linguistically, it seems like a big deal. Someone at the FAA many years ago seems like they were encouraging access to the airspace. 
Yeah, I’m not sure what the thought process is, but everyone talks about two-way radio communication for class C, but it’s also the same rule for class D: You also can’t pop into a class D airport without speaking to tower and having them reference you by name. So really, class B is the odd man out. I guess they want to be extra cautious with class B.
Class B seems intense! I see plenty of YouTube videos of pilots being stressed out in B, or feeling privileged for gaining the access.
Oh yeah!
Here’s a video I really enjoyed where they took their Cessna into O’Hare:
Canadian airspace is more like what you describe for EASA, as well. Class C in Canada is more like American Class B.
I suspect it’s not so much that the various aviation authorities didn’t coordinate, but that the Americans are once again the odd kid out in the Sesame Street skit: 
One of these kids is not like the other, one of these kids is not the same, one of these kids is doing his own thing