Depending if you are planning on adding adjuncts and the types of adjuncts you will need to adjust your mash profile to account for these.
Liquid malt extract/ Dry malt extract/candy sugar/dextrose are all designed to add fermentability to the beer, but in doing so, they can lower the body of the beer leaving the finished beer to dry and alcoholic. Therefore, when adding adjuncts the body of the beer needs to be increased, this can be done by raising the mash temperature or by adding oats and or wheat.
Oats and wheat are added to the mash, but most other adjuncts are added to the end of the boil to increase the OG of the beer or in the fermenter (incremental or staggered feeding)
Brewing high ABV beers can be a tricky thing for many homebrewers. Due to the relatively small size of vessels we use for brewing, we are often limited for how much grain and water we can mash with. As a result, brewing higher ABV beers by merely increasing the quantity of grains that we use is often out of the question.
The basic idea is that you perform multiple mashes, using the wort created in the previous mash as the brewing liquor for the next. Clear as mud?
It becomes more straightforward if you look at it as a series of brewing stages:
1. Start with your initial grain bill and the necessary volume of mash water. You will be mashing the same grist each time you mash. Say you start with 5.5 kg (12 lb) of grain. You will need 18.1 L (4.8 US Gal) of mash water and 14.1 L (3.7 US Gal) of sparge water.
2. Mash in as usual and recirculate. Mash for the full 60 minutes. Pull up your grain basket and let it drain before you begin your sparge.
3. At the end of your sparge empty the grain basket of your first grain bill, put the grain basket back into your Grainfather and begin mashing directly into the wort for your second mash. Once all of your second grain bill is added, start recirculation and mash for an hour.
4. At the end of this second mash, sparge to bring the volume back to 28 L (7.4 US Gal) and start the boil as usual. Alternatively, you could do a third mash at this stage to raise the start gravity even higher.