He should just have to cut the toekick of the cabinet, or your cabinet maker (if custom) can design with them cut out already.
On another note, the best price we found for toekick heaters was at pexsupply.com

It depends on the type of heater. I have a hot water toe kick heater that is welded in place to the hot water pipes coming up from the basement. The contractor had to cut an access panel in the bottom of the cabinet. Wasn't a big deal. Good thing too because some air got in the line and the thing stopped working. My contractor was easily able to drain the heater and get it working again. Just pick cabinets that are easy to empty and not under the garbage!!

With mine, they put the vents in the floor and then cut out the squares from the toekick part of the cabinet for the vent and vent cover. They didn't cut into the floor of the cabinet to do that. I have a forced air system.

Clarify what he means.

Our toekick heater has flexible lines so you can pull it out from the cutout in the toekick to bleed, repair or replace it. Well, theoretically that's what you could do; unfortunately I forgot why I needed to put the cutout in the side of the cabinet and told the carpenter to cut the front. After installation, I remembered that the front of this cabinet has an apron so I can't pull the heater out! Oh boy, if I ever have to fix it I'll have to cut a hole in the base of the cabinet interior after all.

We also installed a circulating pump to be sure water made it into the toe kick heater. I've read in many places that this can be a problem so be sure to talk to your plumber about it.

Make sure one does not end up under the trash pullout like it did in a kitchen I designed :(

We put our kick heaters under our island (one facing the sink/outside wall, and one facing the pantry/inside wall). The toe kicks under the island were cut-out only where the kick heaters themselves sit, and the grills for the kick heaters cover the cut-out (we used the black plastic grills supplied with the heaters, but could've purchased SS look covers from the manufacturer had we so desired).

We have hot water baseboard heat, so the kick heaters run off of that (new lines were run for them). There are three access panels cut into the bottom of the island cabs--one in the bookcase end, one under a small pull-out in the mixer cab, and one under a large drawer along the side of the island. The two under the drawers are only visible when the drawers are removed completely from the island which we've only needed to do in order to access the heaters/pipes (had to do that last year when one of our pipes/welds developed a leak). The bookcase panel is covered by a finished piece of maple/birch which matches the interior of the cabs.

FWIW, we used the Turbonics kick heaters and are very happy with them. Even our plumber, who tried to dissuade us from getting kick heaters ("they're noisy, you won't like them"), was impressed with these after he installed them. We used the supplied switches, and each heater is switched separately, but we could've hooked both up to one switch, or not used any switch at all (and allowed them to turn on or off on their own when the heat comes on). The switches have hi-off-low settings.

The owner (?) of the company wase very nice, and answered our numerous questions before, during, and after the installation. Ours have been installed/in use since 2007.

PS I learned about Turbonics here, and you could say that it was that thread that led me to becoming a member of GW!

Here is a link that might be useful: Turbonics

Mine is just like maria410, hot water, welded pipes, and needed an access panel in the bottom of the cabinet. It is a drawer base so it's not an issue at all. Even if it wasn't a drawer base, you could put a liner on top and you'd never see the cut out. I would definately allow for the access panel now as opposed to later when something goes wrong. They can do a nice neat job if you plan for it.

What type of heat?

I have hydro-air, which to most looks the same as forced hot air (ducts instead of baseboards). The proper way to do it is to have a supply under the sink and they attached a custom piece of duct to turn 90 degrees and bring it to the face of the toekick. The toekick is then cut and a grill is attached. I just have the regular bronze colored grill and it isn't noticeable on my cherry cabinets. I could get a cherry cover made, but less air comes out through the wood grills because of additional resistance (something to know, if you plan to add wood grills the HVAC guy needs to know that so he can upsize your ducts and grills).

Do not let them tell you the hot air can just dump into the space under the cabinet and then eek out through the grill, without an attaching duct. It doesn't work and just superheats your cabinet.

We have forced hot water for heat. I was nervous with cutting the cabinet but I didn't think about the benefit of having the access panel should something go wrong.

I have never heard of turbonics. I will have to find that thread that swayed you cat_mom!! Was it costly? I really like the idea of being able to turn some toe kick heaters off in need be. I don't know if you can do this with any run of the mill toe kick heater. But we are having this sort of heating under the sink and apparently under two different window seats.

I wasn't home when the plumber came. He spoke with DH who, like me, has no idea about this sort of thing. But I think I would have asked more questions at the time. I'm thinking the plumber is planning on buying the materials. Is there anything that I should make sure he doesn't get?

I don't think ours were particularly costly. I called the company and described our set-up, and the size/length of the baseboard we were going to lose with the reno. The gentleman with whom I spoke (Craig?), told me which kick-heater/size would be best for our application, and gave me the name of the rep who supplied our area. I then got the name of a local distributor/retailer and went there and got them.

I liked what I read about the Turbonics heaters on their site, and I liked them even more after I called the company. The woman who answered the phone was genuinely impressed with the product they manufactured, as was Craig (owner? I think). IMHO, they were sincere (I had a gut feel about them) and they just knew they made a quality product. I felt we'd be in good hands with them--after-market support and all. We had no prior experience with kick-heaters, but knew we needed them for the new kitchen, so decided to go with them based purely on comfort level with the info about the product and the company itself.

Another FWIW; in the original thread, the OP or a follow-p poster, stated he was a plumber, and he only installed Turbonics kick-heaters. That seemed like a good endorsement right off the bat. I tried to find that old thread for you, but came up empty-handed. Sorry...