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Question DetailsAsked on 12/1/2015

How much should I anticipate spending to dig out 2' from a basement that's approximately 900 sq ft of existing home

I going to be purchasing a semi detached corner unit home in DC that was built in 1923 and has about 900 square feet of basement that current has about 6'7 ceilings. I would like to dig out the 1 foot to 2 feet of the basement floor to achieve ceiling heights of around 7'6" to 8'6' to make to the basement a usable dwelling unit. What are some of the concerns I should have with doing this work (such as settling or moving of sanitary drain pipes) and how much should I expect to pay for doing all of this work?

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First thing would be to check with planning and zoning to see if you are allowed to do this or not, and what types of living space you are allowed to put in the basement under local code.


You secondly need an Architect with connections to a structural/civil engineer with structural and foundations expertise (or a civil engineering firm with both) to determine your foundation situation and prepare both structural and construction sequence plans - because dig down and remove the supporting soil next to the footer of a foundation and houses tend to fall down. In many cases, this sort of floor lowering has to be done incrementally or only after anchoring the foundation with soil anchors. Commonly they will remove the soil in strips and cast concrete beams across the basement to tie in under the foundation footers and prevent them from kicking in, then remove the rest of the soil and place base material and any drains, then pour the new slab over and connecting to the beams - or sometimes put the slab in in strips, doing maybe half the area at a time but in alternating strips of concrete with original soil left in place between, then remove the intervening strips and concrete them.


Certainly any pipe that will be within a foot or so of the bottom of the new slab (and remember you commonly have to overexcavate 6-12 inches to put in drain or base materials under the slab) should be deepened or relocated during the work. Water pipes I would bring up into the ceiling if possible - gives you brand new piping for that section plus eliminates risk of having to dig through slab if they leak in the future. Sewer the engineer will have to survey - depending on elevations, etc you may or may not be able to drop it and still have proper slope, or may (commonly) have to drop at least some of the outside pipe as well to provide adequate slope from the lowered inside pipes. In some cases dropping the pipes would put them below the outlet to the sewer or with too little slope, so you have to replumb so the drain pipes go through the basement ceiling from the ground floor and above, and any laundry tub or bathroom in the basement has to go to a sewage grinder/lift pump to be pumped up to the drain pipe.


Depending on your situation, dropping the floor elevation might also require rebuilding basement drainage facilities - certainly any underfloor drains or sump pump system, and sometimes outdoor french drains along the foundation that are above the new slab bottom elevation so they would not lower the water level below the slab, necessitating a different or revised drainage system. In fact, in some areas goiing deeper with the slab might get you into the permanent or seasonal water table, which could result in an on-going pumping need and both high maintenance and electric costs, not to mention a potential mess during power outages or in the event of a pump failure. This can become critical in areas where sump pump discharge cannot legally go into the sewers if in an area with freezing conditions, so you have outdoor discharge of water on the surface causing glaciering and pump discharge pipe freezeup.


On remodels (as opposed to new construction where high water table can more easily be accomodated) I always recommend that unless it is possible to drain the foundation excavation to free air by gravity with french drains or daylight basement, not to deepen the bottom of the slab closer than 1-2 feet to the highest normal seasonal water table.


HOW you lower the slab matters a lot to cost, too - it is much costlier to extend the foundation downward the foot or two while installing the slab than it is to leave the foundation the same as it is, and installing the slab so it has a raised structural support edge that ties into the inside face of the existing strip footer - essentially leaving a concrete ledge a foot or foot and a half wide all around the room that would typically have a height of about 6-18 inches - which can be used as a base for cabinets or bookshelves or such, but really eats into the available basement floor space.


Also - you said 1923 home - so if built with poor quality or deteriorated concrete, true cinder block (as opposed to concrete block), solid brick or stone walls - that can affect your plan too, because they do not take as well to being undermined to deepen the foundation as poured concrete or concrete block, so the work has to progress in smaller pieces to avoid breaking up the foundation.


So - can end up being more than you expected at times - definitely should have an Architect/Engineer in on the planning and estimating, before you get into a contract with a Foundation Repair or General Contractor. Though having contractor on board during the planning stage can be helpful if you are selecting one based on reviews and recommendations rather than by competitive bidding, so what you plan to do (with his input) is something he is able to actually perform on economically. In residential, as opposed to larger commercial jobs, sometimes the technically "cleanest" solution is not the most economic for a small contractor to do because he may not have the experience with the technique or the special equipment (like a highly portable drill to set anchors) to do the job as planned, or it may depend too much on him getting everything right in the course of the work. Temporary support and excavating under foundations are two of the areas where this factor is critical and results in a lot of Oops moments.


Cost - the actual dirt removal could run probAbly about $2000-5000 depending on accessibility - it dirt removal by wheelbarrow to a bobcast parked at a basement door, by conveyor through a window, or hand carried by bucket up stairs to the yard. And how hard digging is it, and is dewatering required, etc. Of course, if bedrock - then can run up to the $5000-10,000 range depending on hardness. The slab itself about $3000-10,000 probably depending on how "structural" it has to be and whether it has embedded beams or not, and whether it has to be. Concrete underpinning cost varies widely depending on a lot of the above factors and the strength of the existing foundation, and of course on the design (whether it requires extending the foundations down or not and if they have to be tied back) - so could be from zero to maybe $10,000 or more for that. And probably $1000 or two for plumbing mods - maybe a couple thousand more if need to realign the grade all the way to the street. Plus probably a few thousand minimum engineering and permits and such - possibly twice that for more difficult foundation conditions.


And since you said semi-detached house - the foundation and basement redesign will have to take into account the support for the adjacent structure(s) too - both foundation support, risk of settlement damaging the connecting structures, and the risk of adjacent structure losing support due to lowering of the water table - either during construction dewatering or permanent drainage system, especially if you are around Foggy Bottom or Anacostia or similar soil conditions.


So - like many foundation rehab/improvement jobs, you get a VERY wide range of possible costs - I would figure from the above about $9000 bare bones and highly likely in then $15-20,000 - and possibly, if everything is going against you, as high as $30-40,000 or in unusual cases even more.


I did not bring up the possibility of raising the building on the foundation because of the semi-detached factor - but in many cases that is a viable way to increase basement headroom, especially with rectangular buildings and in cases where foundation conditions or the cost of deepening the foundations is very expensive or difficult due to poor foundation conditions or construction.


But - realize this is all sight unseen - so you need an architect and engineer to evaluate the conditons and give a more accurate project cost estimate - and if this is something you are figuring on as part of your decision on how much to pay for the house - you should have a firm bid from at least one contractor good till well after the planned closing date, so you don't end up with an underestimate. One thing on that - thjis type of job overruns are common - lifting a building much less so because the conditions are well known, so my gut feeling for this type of job - aside from potential difficulties regarding connections to the adjacent building which might easily rule it out - would be raising the building versus lowering the basement.


One other factor if not committed already to that property - you are likely to get a full height basement in another comparable home cheaper than having it done yourself in this one, because your typical value recovery factor for upgrading an existing basement is typically 40-50% or less.

Answered 2 years ago by LCD




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