NOTE: This advice is not 'expert.' If you are unsure about the legal situation, consult your insurer, lawyer or the relevant organisation
House Concerts are a recent import from the States. Some are organised by an artist's fans to secure a private gig (like this). Some run on a more permanent basis, booking a range of acts but without the formality of a full folk club (like this), and some are 'actual' folk clubs which happen to operate from a private house (like this). (And a number of singarounds now take place in people's houses too). More here
They're essentially a cross between a party and a gig - and very easy to arrrange.
SIZE AND NUMBERS
The first requirement is a large enough room. How big will depend on how many people you want to have, how much you think you can charge, and how much the artist asks for! Some performers are happy to play a house concert for less money than a normal gig as HCs do offer advantages over public events. These include more reliable attendance with no promotion costs, cosy atmosphere, no venue issues, no unwanted people, food, on-site accom, and an ability to run the night as you and they prefer (with support, spots, session or nothing).
15 at £8 a head may be suficient for a solo gig (TB has done this when travel and other issues permitted - in a 'typical' small suburban semi). 20 at £5 might work for some people, though 20 at £8 would be better, and an ideal (from the artists point of view anyway) might be 30 at £10.
So if you can make room for 15+ chairs (and/or cushions if the audience is young enough), and a bit of space for the performer/s? If not, maybe you could think about an open air event (in summer only - and even then some cover would be wise!) or involving a friend who has a bigger house.
LAW AND MONEY
As we understand it, to avoid the need for PEL , PRS licences and other similar issues, money must be donated directly to the artists on the night, not collected by the host as a set payment on the door (though you can collect the donations in advance if you want to be sure they'll turn up).
Another idea would be to have free entry but a 'strongly reccommended' purchase of a CD instead (you can always sell it on if you've already got them all). That would be legal. And there's always the good old 10 Gallon Hat.
You need to think about insurance and health and safety. As we understand it, if the money is being donated, then the event counts as a private party so laws governing private events apply.
The situation is somewheat unclear (as it seems to be in the USA too), and may only be clarified by future court activity. If in doubt, seek advice from your insurers or an independent expert.
Even if the music will be quiet it's a good idea to check with the neighbours first - with luck they'll be your first 'customers.' It's certainly a good idea to avoid disputes which might bring in the authorities.
Remember that parking could be a stumbling block.
The artist/s will probably require an evening meal (often we prefer to eat after the show) and acommodation, as House Concerts don't usually generate enough revenue for BnBs.
You might want to think about providing an evening meal for the visitors too (in the break or after the show - maybe as part of a party) This might affect donations/prices if you plan to recoup your costs. It's best not to try to include a full meal, unless you're really sure you have the capacity to feed everyone quickly - otherwise the food can wind up dominating the evening. A gig with drinks and snacks is the best approach.
Your visitors will want a drink. You can either arrange a donation-only bar, or have people bring a bottle. But don't serve strong liquer - at least not till after the show. Most performers will probably prefer to play to a (relatively) sober audience! If you want to have a party afterwards that's fine, but bear in mind that they may be on tour so may not join in for long - plus they'll have 'given their all' at the gig and will probably just want a couple of glasses, a nibble and then a nice soft pillow (somewhere away from the noise)!
Parking may be an issue - make sure you have room for those not arriving on foot (neighbours are a great place to start) or a car park close by. Artists may need to park close to the house to bring in instruments etc.
Advertising is best avoided if the event is to be proved to be a private party. Also, you'll need to know who's coming, and trust them to turn out. You don't want people turning up and being dissappointed, and you don't want any empty seats if space is already at a premium. It will be harder to manage the numbers if the event has been widely advertised.
You'll also need to liaise with the artist/s to see what promotion they would like to do. For example, should they list the gig on their web-sites, should they mention it to their mailing lists, and if so how much information should they provide? This might range from merely indicating that they're not available for a booking that night, to providing full contact details. It will certainly help if they can estimate how many they think will come, and it's advisable to ask them to provide a phone number, but NOT an address, so that people will have to ring for directions, allowing you to manage the numbers.
So, ideally, invite the audience personally, but only invite the right number. Ask people you invite to think of others who might enjoy the show, but try to make the contact yourself it you can.
And do ask people to let you know if they have to cancel, so you can find replacements. This is not like a party where you expect a few no-shows. You'll want a full house, but no more.
Make sure everyone knows how much the donation will be, where you are, when to come (30 mins before the start is usual), where to park, and what to bring (drinks, food, folding chairs etc).
You can include floor spots or supports or a session or singaround as you like. Two 45/50 minute spots from the artist, with a nice long break to chat is a good start.
If your house concerts are going well, you might want to think about expanding to a larger house, or even to a hired hall or marquee. If so, and the event becomes pubic you'll need to consider things like licences, insurance, tax, promotion etc. so may want to look at the item listed in the menu above. (And some of those issues might be relevant now anyway).
The UK Folk Performers' Network