How much does it cost to dig a basement in Boston (Somerville)?

Question

I just found out that due to the new Somerville zoning I can build an addition onto my two family in Somerville!

This would be about 600 sq feet. The next thing I’m trying to wrap my head around is do we dig out a new basement and try to make it liveable? Or just go for a slab? I don’t know if a slab foundation is allowed in Somerville just cause every house has a basement... And would you connect the new one to the existing one?

Anyways my question is how much would a basement/ foundation, for what is essentially a new build cost, so that I can figure out if it’s worth it? 600 square feet  here...

I know the first response is “well, it depends...” I know it does but just do your best to give me a ballpark for the Boston area. Or even just your area wherever that is? I can’t find even a slight idea online. How much is it just for the foundation and then to finish and make liveable would probably be like at least 60k with HVAC and everything right?

Thanks so much for any advice/ info/ or direction here :)

Ian Martin · Answer

Yes, it depends, but to answer your question, yes, at least 60k. How far are you digging down? That's a big factor. How far below grade does your current foundation go? Another big factor. There is really no way to estimate a number without seeing your current circumstances. You need to get contractors on site in order to get a proper quote for everything.

Tony D. · Answer

@Adam Sankowski

What type of foundation? Careful with Fieldstone.

City engineers likely to insist on water infiltration system if you go below water table.

I was told a proper water table measure would take time. I would love a soil engineer to chime in here.

If you’re going thru trouble of breaking existing slab to dig down then I’d go further than just 7-1/2’ depending on water table.

Also, where exactly in Somerville? Ton of drainage work has been done in union square which should help with flooding and I hope lower the water table.

Ryan Wittig · Answer

Hate to say it, but IT DEPENDS a lot! If zoning permits you to build another detached structure, you should definitely do that. You can certainly do slab on grade, but the basement space doesn't count towards your livable sq ft if you don't have a bathroom or bedroom. It could be a media room, exercise area, kids play area and storage, which are all valuable. If you need to connect to the existing building this will have significant structural implications for your current building. If you are attaching to your existing building, your new basement can not be deeper than the seasonal high water table, which may affect your ceiling height (doesn't apply to new construction with certain requirements). You are going to want to know the water table height regardless if digging. You're also probably going to want a geotech engineer to look at your soil to determine if any remediation will be needed to build a new foundation...

Is 600 sq. ft. the footprint of the building or total sq. ft. that you can add. If it is, it sounds like you could build a roughly 1800 sq. ft addition, correct? Assuming 600 sq. ft. is the footprint your basement would roughly cost $25-50K to complete which consists of at a minimum digging the hole, creating the proper draining and pouring the foundation.

Interested to hear if anyone else can fact check me or give their experiences.

Good luck

Ryan Wittig · Answer

I agree with Ian's number. $60K is more likely in the ballpark. If it's up a hill the water table probably isn't a concern, but you will still need to have someone validate where the water table is. Somerville does not want people digging down too deep and using sump pumps to drain the earth around them.

Not sure how to answer your question about how the buildings and foundations are connected. Talk to an architect. Khalsa and Peter Quinn Architects are two popular shops we've used.

Get a contractor out there who can give you real numbers.

Tony D. · Answer

@Ian Martin

Yup

Underpinning or a knee wall

Adam Sankowski · Answer

These are all great things to bring up with our architect thanks!

Ian Martin · Answer

What @Ryan Wittig said, have a contractor come out who can get you real numbers. You're not going to find your answer on here unfortunately. If you're already 7 feet down in your basement, that's a good start. You will need 7 and a half feet to make it habitable living space, but you'll need to dig down farther than that to account for ceiling and floor heights and utilities therein. This could very well mean underpinning the existing foundation. If you're very lucky, your existing walls will go down far enough to account for the added height, but it's unlikely.

Just a heads up, if the city deems you need to upgrade your service and utility lines to account for an additional unit, and/or add sprinklers and/or add a fire alarm system, this is going to cost you quite a bit. Again, get some contractors out to get you going in the right direction and give you real bids. Best of luck.

Adam Sankowski · Answer

Hey @Ian Martin and @Ryan Wittig Thanks for the info and to answer questions:

- I don't know about the water table but it's about half way up a hill if that helps?

- It is not a separate building but this would be attached to the current house- an addition off the back.  So I can add 600 sq. ft to both the first and 2nd units.  It makes sense that there would be a lot of structural implications for the current building.  Is it possible to leave a few feet between the two so that the basements aren't actually attached to avoid this issue ?

- Current basement depth is right around 7 feet. I would only want to do this if its liveable so I guess it would have to be at least 7 feet deep.

Ok at least 60k is a number :) And that's just for a poured foundation nothing else?  Thanks guys!

How much does it cost to dig a basement in Boston (Somerville)?

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