"Not 99%, we need 100% odor-free"

Well, then you'll have to replace everything. Not refinish. Replace. If the prior owners were not keeping the place up, there is likely urine soaked into every surface. Walls, baseboards, trim, floors, subfloors, cabinets etc. Anything that liquid can damage will have to go if you really mean 100%. Depending on the finishes, you could easily spend more than $100k doing that.

And for smoke, possibly the ceilings, too.

Hopefully any urine stains will be confined to the floor so be sure to have the subfloor and baseboards carefully checked before putting down any new carpet (and pad). If you have stains in concrete floors, you can clean best you can and then paint over them with a shellac to contain the odor.

You will also want to replace all soft window coverings (curtains) and I would consider getting the air ducts cleaned. Wash all the windows. I think you'll likely want to prime the walls before painting.

As a fellow purchaser of "the cat lady" house, I can assure you that smells are not confined to soft surfaces.

Just an example: my house has a fireplace with a ceramic tile surround that extends about a foot in front of it. We had taken down all the walls, trim, ceiling etc and had the wood floors sanded and refinished. It still smelled when it rained. The urine had soaked into the grout of the tile on the fireplace. Yuck! You would think that at least the tile wouldn't stink, but no.... No amount of cleaner got the smell off so we had to replace all that too.

You will need to do a complete gut of all interior finishes. All the flooring, sheetrock (ceiling and walls) any tile, all cabinets, and maybe even the fixtures. That will easily top 100K. Just refinishing things doesn't get into the subsurface and subfloor. You will probably have to replace the subfloor as well.

But first I would try an old fashioned top to bottom scrub followed by an enzyme spray like Nature's Miracle and really soak all surfaces well. Then repeat. Then try an ozone generator it it for a week or two. See what that does. IT may surprise you.

On the other hand, if you want 100% of the odor gone, you may want to bypass this house as a risky venture and possible financial black hole.

You might be surprised by what Live Wire suggested with the top to bottom clean. When we moved into our house the PO has used one of the bedrooms as a cat (litter) room. YUCK!!! Carpet reeked. We didn't have enough money to replace. DH practically soaked it with Odo-ban (we've used this for years on our dog kennels) and opened the windows. Surpisingly it took the smell out. My son actually lived in that room for quite a few years. 7 years later we pulled up the carpet and I refinished the floors and they weren't as bad as I was expecting.
On the smoke issue - my Mom was a Big smoker - 40 years in her house (closed up). When she died we had to clean. Sad to say she had stopped cleaning a few years before she died but she never did some of the major things. The walls, appliances, everything "dripped" brown when you put a rag to it. We (mostly DH) put hours into cleaning and then repainting, new carpet & floor. By the time we were done - no smell at all.
It was alot of work but something we had to do.
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You can call Serv Pro to get an estimate. They clean homes and fix when all kinds of disasters and other stinky and dirty things that most folks won't touch.

Maybe you hire them for some things. But at least they will itemize what needs to be done. And they probably will tell you why.

I read before that good primer on walls really works wonders.

Sounds like a great deal and worth what you are doing.

We bought a house from a heavy smoker. We removed all window coverings & carpets first thing. Tons of cleaning, paint & all new floor coverings later, there was still an odor when you opened windows. It was the metal screens. It gets everywhere.

We bought a house from heavy smokers who kept their house pretty clean. We thought we could get rid of the smoke smell by cleaning everything really well and airing the place out, either cleaning the window treatments or getting rid of them, either cleaning the carpets or worst case replacing them.

Turns out we had to clean and air the home, scrub the heck out of everything, replace the nearly new carpet, throw out the window treatments, repaint all the walls and ceilings, and replace some of the drywall.

Also, the smell of the cigarette smoke masked the musty odor of the home. Once we got rid of the cigarette smoke, we had to find the source of the other odors and fix those. I don't think I will ever, ever buy another house that doesn't smell fresh unless I have no other choice.

Our neighbors bought a home from owners who had a cat who urinated on the floors. They wound up having to replace the subfloor. I think they said they'd never ever again buy a house that smelled of cat urine.

I hope whatever you decide works for you!

Before purchasing, you might want to read Jen's 1926 Boston Farmhouse; or The Smelly House Blog. Jen's has a lot of experience with a smelly house.

Here is a link that might be useful: The Smelly House

Thanks to all for the helpful hints. I am scared to death of this house and think it might be too much of a black hole for me. The smell is absolutely overpowering - serious freak-show type stuff must have been going on in there.

I think we are going to pass and let someone else deal with it. In the end, it might take a bulldozer to make the house habitable. I'm not financially prepared to do that. The comments here were helpful in supporting my decision. Thanks

You should meet with ServPro on site for a quote before you walk away. They are a franchise in most states. They know how to find the source of the odor and remedy. You might find that they can identify/resolve for cheaper than you think. They have a web site and it lists the franchisee for your area.

Can you have your realtor take you for a showing and schedule ServPro to come out am meet you and take it from there?

And if still on the fence , offer 200k lower than what you planned to. Would it be worth it then? Of course it is only worth it if the smell can be gotten rid of without bulldozing.

We bought a home that probably smells a bit like yours... some thoughts:

The pet smell has been easier to clean up - replace carpet and replace underlayment that has pee stains.

For smoke You'll need more than paint - you'll need an high odder blocking primer. I recommend Zinser BIN because it works in one coat but it is just awful to work with. Walls, ceilings, trim, everything will need to be primed and repainted or replaced.

The smoke was in our kitchen cabinets. It's also in the blinds and window molding. I could smell it on our exterior door and can still smell a bit of it on our front porch on certain days.

We haven't replaced everything but we have replaced/painted most big-ticket items. Generally it's not noticeable and people don't claim to smell it, but on certain days if you stick your noise in the right places it's there.