I currently have two 6 volt (220 amp amp hours) deep cycle batteries for the house bank and one 12 volt AGM (105 amp hours) for a starter battery.
How are they charged? I presume the 6V batts are wet and you said your starting battery is AGM?
I would love to have the room to add two more 6 volt batteries but do not have the space.
Common issue. How deeply are you currently discharging your 220Ah bank? Do you have a battery monitor to know for sure how deeply you're discharging?
Should I settle for the 220 amp hour battery bank
This can't be accurately answered unless we know your average depth of discharge and have more information on your use.
or move the AGM starter battery somewhere else
Always an option if you need to.
and add a 12 volt deep cycle battery to the house bank?
That would not be the best choice. As others have said batteries wired in permanent parallel banks should be installed at the same time, from the same date code, same lot # and same model. All batteries have slightly different internal chemistries, that also change with age. You want to wire contiguous banks with all the same brand, model and age batteries for the longest life. They also need to be wired correctly and loads should be pulled across a bank not off of one end. I take it a step further when buying batteries for customers and myself and put an $800.00 analyzer on them to find the best matches to put into a contiguous bank.
My electrical needs are modest for day sailing but I would like to do long weekends next year.
The best money you could spend at this point may be a battery monitor. The Victron BMV-600 is an excellent value and can SAVE you money..
Personally I would not make the switch, for your described use, to AGM or GEL. Most every AGM maker, Lifeline, Deka, Trojan etc. suggests a max depth of discharge of 50%, just like wets, and even at 50% DOD I find AGM's generally speaking still last less long than good old deep cycle wets do.
In the beginning AGM and GEL makers did suggest, and in some cases still do in marketing materials that don't tell the whole story, that AGM & GEL can be more deeply cycled. In reality they found out that was just not the case. GELS will still get more cycles than AGM when deeply cycled but you are paying up to 3X the cost of wets that will come darn close and should easily exceed 5 years use..
GELS can certainly be the longest lasting batteries but also the most difficult to keep properly charged and when compared to a decent quality deep cycle wet battery like a Deka/ East Penn, Crown or US Battery they cost about 3X as much.
Switching to AGM and GEL may require a large upfront expense on not just batteries, often 2 to 3 times more than deep cycle wets, but charge regulation as well if you don't already have it. Charge regulation & alternator upgrades can get very costly especially when added to the cost of a bank of "gourmet" batteries.. For "occasional weekend use" I find this money could be better spent on conservation such as LED lighting, or, if applicable, dedicated 12V devices as opposed to using an inverter..
If you can get us some more usage info and we can better assist you in getting the most bang for the buck in the space constraints you have. You may find that 220Ah's, or more accurately, 110Ah's, is enough for what you want to do.
If you find you do need more Ah's, 3 group 31 Deka deep cycle batteries with 105Ah each, or 315 Ah's for the bank, can be had these days for under $250.00 for all three batteries. With some 12V group 31's you can get as much as 390 Ah's with just 3 batteries..
Figure what you need first and consider a battery monitor, under $190.00.....