I bought a 1927 house last year and, afterwards learned I'm especially sensitive to mold and this house has plenty of it. I've eliminated mold everywhere I've found it except in the inaccessible crawlspace under a kitchen and bedroom addition (1960s? 70s?). I've fixed leaks and replaced drywall, ductwork, etc but I can still smell mold coming up from the crawlspace, which has no access panel but several vents. The height ranges from about 10 inches under the kitchen (worst mold area) to 2 feet at one bedroom corner. I don't know how to get rid of the mold under the addition: seal it and find a way to keep it dehumidified? pay someone to spray foam underneath? install ultraviolet lamps somehow to kill the mold? Help!
You can get lower end foggers that just need some gap to point them in for about $60-$75. Probably less for a smaller area. I was pricing something that would hold a full gallon of solution and do our entire basement. Mold will be dead without having to get to it but only temporarily if the space remains too damp.
Microbalance ec3 is an awesome nontoxic mold killer for immediate relief from mold problems. It is very temporary though and costly to keep fogging a large area. You can also get ec3 candles that give off no scent but kill mold in a very large room if burned for at least 3 hours. Tested by several mold remediation professionals using their own preferred labs. It can eliminate your misery for weeks to a couple months depending on how fast the mold comes back while you work on the problem. Slight caution with pets for the nontoxic part since they do not process plant oils as well as humans. High enough chronic exposure can cause health issues but periodic exposure at lower levels is not a problem. I just make sure the cats don't get in the basement until it all dissipates from the air and use the lower concentration candles or spray in the rest of the house no more than twice a week.
Concrobium is another nontoxic mold killer that works by mechanical means and stays on the surface so it will continue to kill new mold but it is not resistant to water. It needs to dry some to be fully effective and it will dissolve back off with enough new moisture. It is an irritant if you breathe too much but it's just a mixture of salts like baking soda and washing soda so the one time I fogged with it I just wore a basic disposable mask to reduce how much I breathed in. Didn't have any problems beyond drying out the skin on my hands.
Long term if you don't want to be fogging every month or 2 your only solution is to dry the area out. You'd need someone to look at the details of the space to tell where ventilation or a dehumidifier could be installed.
After killing the existing mould. Ventilation is the answer (usually).
I'm rebuilding a stone wall/underpinning in a large underfloor area (appx. 1.7m high or nearly head height) and it had a serious mould problem a couple of years ago. The guys came in, cleared out a load of old mould infested rubbish and then installed an air pump with a series of metal ventilation ducts with rubber outlets.
The system measures humidity and temperature and then draws in air from outside and pumps it through the ducts into the whole basement. I think they aim for 50 to 60% humidity and around 10 to 12 degrees C down there.