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I can not really tell without seeing the site but I think I have an idea as to the topography of your lot. If as you say it sloped away enough that you could have a walk out basement you probably will have to excavate to below the frost level at least at that end and then step the footings up as you go up the hill to your existing house. I think there will be minimal added expense since stepping the footings and blockwork adds labor costs to the project. You have to put a vapor barrior down over the crawlspace area and around here you need a minimum of 2 inches of concrete to protect it by code. I have never used 2 inches but use 3 inches because I feel it is easier to spread out, so for a full floor if done the way I do them you are just adding another inch of concrete and the cost for a better finish. I have always asked most customers if they would want the added storage and have said that you could not build a decent shed for the added cost. It has been awhile but I would think you are adding about $1,000 to $2,000 to the cost.

Don

By the time the added cost of the footings are factored in I doubt there would be much difference. Also, any price increase would more than be made up in your home's added equity/value in this case. My suggestion is to speak with a structural engineer about your options and have the plans drawn up. You'll need one for the plans regardless before you can get bids on the project so you might as well start there and know all of your options with their approximate cost differences. You could also bring a reputable builder/contractor in on the conversation so everyone is one the same page from the start. Engineers often have an idea what something might cost but aren't in the business of keeping up with current pricing, especially if it is out of their typical market area. S/he will be able to give you a rough idea but don't expect it to be spot on when you go to bid it.

A walk-out basement would certainly add to your home value if you can afford it, especially if it has ample windows on the low ground level side and you put a roofed-over patio in front of it - which could be a roof, an awning, a solid deck on the next story up, an overhanging upper story on the addition, etc.

The two prior comments are good advice, but seem to assume your foundations would be going to full basement depth anyway - i.e. that your frost penetration depth is enough that it gives you a full basement.

If you are in a part of the country where shallow burial or slab on grade is the norm and that is what you have, then a half to full UNFINISHED basement depth (3-8 feet below ground surface) is likely to add in the ballpark of $20-40/SF to your cost depending on added depth, for the extra excavation, waterproofing and maybe subslab drainage, basement slab, and additional foundation wall height. To finish it (go from bare foundation walls and slab to something that is a living space) can run from $20-100/SF depending on whether you are talking just painting foundation walls and some lighting, simple economy finishing, or fully finished and conditioned space. Certainly closer to or even above the $100/SF number if the basement space is designed as a legal bedroom space with full bath.

One emphatic recommendation - when you do your addition, and most especially at the foundation, require that expansion seal be used at the interface of the walls and roof, crack control joints in all walls and slabs, and that the foundation wall and slab where it meets the old house be built with a crack control joint and waterstops, as you WILL get differential settlement and dcracking betweenthe addition and the old part of the home - the key is to control where it occurs, and provide sills and trim strips and such at those points so it does not cause random, wandering unsightly cracking. Generally, your best bet is to force the crackingto occur at the interface between the two, where it wants to occur anyway, and at the roof bridge the roof over the joint rather than making a sharp rigid transition right at the seam, that way the differential settlement might slightly slope the roof in that area but not cause loss of water shedding capacity or tearing of the sheathing and roofing waterproof membrane. Another common way to avoid the settlement sissue at the roof is to make the new addition roof a different height than the existing, so there is no actual roofing "joint" at all.

If you live in an area that allows slab on grade foundations it minght actually cost about the same or possibly even less. You said the ground slopes to where you would have a walk-out basement due to the extreme slope in your yard. If I am correct by saying expanded basement I assume you have one now so in essence you would just be adding a thickened edge slab type of foundation and then building a block or concrete wall on that. This would eliminate having to excavate to below frost level at the exposed end and stepping the footings up as you work into the hill. Even if you do need to go below frost level the only area you have to do this is at the end by the walk-out and into the hill in steps to keep the sides below the frost level untill there is enough soil to protect it from frost heave. You can get an idea as to this by looking at your hill and thinking about how far into your hill you would have to dig into the hill to be 3 feet below grade from where you are standing and adding a few feet and then adding a few more feet for the steps needed to come up to the grade your are standing at. Those steps generally are at 8 inch increments to match the size of the concrete block. A crawlspace basement would need the same steps with the same walls plus back filling and compacting the soil as you go. I think the only difference would be the need for 12 inch block for the whole wall as apposed to possibly being able to use 8 inch for the areas supported by soil on the area higherninto the hill and the fact that you will need far more steps to come up the 3 or 4 extra feet. You can also get an idea by looking at how much foundation is showing at the rear of the house. If you have about 3 feet showing you will have to go down about 5 feet plus the footing to get an 8 foot ceiling in the new basement area plus what you need to maintain frost protection at the exposed end.

Don

Good comment by Don - one additional recommendation - if you do a daylight basement or hillside cut-in half basement, take the $1000 or so extra bucks to put bitumastic waterproofing on the basement walls, vapor barrier under the slab, and install a perimeter french drain a foot or two below the bottom of the footer exiting to daylight, to eliminate future damp basement slab or water leakage into the house. SO much cheaper to do during foundation excavation then after the fact - and of course remember to grade around the perimeter to rainfall and gutter runoff drain away from the house.

Remember, even if this is going to be a workshop for you, to a buyer when you resell, they may be looking at a fancy den or entertainment room, so any signs of moisture issues in the basement could scotch a sale.