If a character has a spell potency, the rules say that they can design potent spells within their potency. Does this designing of spells only concern formulaic magic, or are spells cast with spontaneous magic also "designed"?
I think that a sponteneous can't be designed... because if designed, it's not spontaneous!
But I'm quite a new player in ArM.
I would argue yes, sort of. A focus applies to sponts, so potent magic should too. On the other hand, I think that since incorporating casting foci into a spell should be less than instantaneous, it should require ceremonial casting.
Mind you, I have issues with Potent Magic as it stands anyway, so ...
ETA: Having checked the book (HoH:MC, page 6), it explicitly states that Potent Magic's S&F bonus cannot be used in Spontaneous magic.
You're not alone (and truth to tell, we all feel that way sometimes still!)
Depends what you mean by "design". A spontaneous spell is one that has not been learned, permanently, as a formulaic (or ritual) spell. But any spell, spontaneous, formulaic or ritual, has to be "designed" at some level, whether done over seasons by a Mage or at the table by a Player, by taking the Base Effect plus any increase in base Target/Range/Duration & etc.
For instance, to light a fire, one takes the Creo Ignem guidelines (AM p 140), finds the Base Effect they want, adds a Range, and does the quick math. To light a wick or lamp oil, it's Base 2, continuing up to the difficulty to light something like wet wood, which is Base 5 (or higher). Add +1 magnitude for Touch (or whatever is desired), and you have designed your spell effect. If that's spontaneous, it's cast differently than if it's formulaic, but it's the same effect.
I used to keep a cheat-sheet of a couple dozen common, quick and dirty base Spontaneous spell effects, so I would not have to slow a game down to design them when needed. Not as complex as some spells, but the process is the same.
Yes i do that too, but for the potency (my future mysteries ^^) i think there is a need to have "created" (meaning learned) the spell with some S&P in it.
So it cannot be the case for "spontaneous".
I have a sheet too 
with my high useful spontaneous spells. great minds things alike 
if they have the Virtue "Minor Potent Magic (wolves)" then the magus has a fixed +3 bonus to all magic (spont/formula/ritual) involving "wolves", and also can devise and modify Formulaic & Ritual spells to be Potent Spells which then have mandatory casting objects and have a Casting Score bonus. Any magus can learn and cast a Potent Spell from a Lab Text created by a magus with the Potent Magic Virtue, if their MT >= spell's potency.
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any spell they cast (spontaneous, formulaic or ritual) which involves "wolves" get +3 on the Casting Score (+6 for Major Potent Magic).
This is a direct parallel to "Minor Magic Affinity (wolves)", and only magi with Potent Magic (wolves) get this bonus, but they get it for all spells involving "wolves" regardless of how the spell was devised, Potent or not.
Using this +3 casting bonus does not make a spell a Potent Spell. -
they can devise new Formulaic spells, and into that spell design they can work casting objects, to make a Potent Spell.
A Potent Spell grants a Shape & Material bonus to their Casting Score (this bonus is the spell's "potency").
A Potent Spell can only be learnt if MT >= potency
A Potent Spell can only be cast if the casting objects are to hand (touched).
Only a magus with Potent Magic can devise a Potent Spell without a lab text or teacher, and only a magus with Potent Magic can vary the potency.
Any magus can learn (invent) a spell with that specific Potency (specific casting objects and MT requirement) from a Lab Text or Teacher.
The S&M bonus applies to all magi learning from a specific text or teacher, and is independent of knowing Potent Magic, and independent of the fixed +3 or +6 bonus for having the Virtue.
- if a magus has the a Potent Magic Virtue covering the area of a spell, they can (quickly - 1 season) devise alternatives with different casting objects and bonus (or none).
Of all those, only (1) the applies to Spont. Magic - the fixed +3 bonus to Casting Score for any spell within the area of the Virtue.
You get you Potency bonus, +3 or +6, whenever casting such a spell, be it formulaic or sponatneous. You also get this bonus to your lab total. Incorporating Casting Items requires a Formulaic spell designed with said items, though I can see a reasonable argument for allowing them for Cerimonial spells.