wow what a nice looking home!
you have good taste........and expensive too!

I'll see if I can come up with a ballpark figure for you based on this plan, plus adding another bedroom.

That design is 4068 sq/feet or 378sq/m......pretty decent sized!
And you are planning on adding a 4th bed to this, so add another 20 sq/m ballpark for this and the size is 398....lets say 400sq/m. You say you will modify it all a bit, but i doubt the overall size wont change much.

ok, lets apply the sq/m rates. If we run with archicentre's $2000 per sq/m, that comes to a princely sum of a cool $800000.

ok for this part of my pricing guesstimate, I am going to use exactly as those piccys shown, except the stone as previously posted, and I'll exclude using the beautiful (but expensive and upkeep cost hungry) stained timber external cladding, but the windows I will include as is, cos if you were to change them it would ruin the ascpects of the house.

anyway, on the window note, windows are one of the most expensive items in a house. And even more expensive, is custom sized ones.....anything non-standard and the costs start to rise. If you use standard sized windows, with standard glazing (ie not double glazed or anything fancy) a ballpark figure is $1000 a window. Then the order of the most expensize is louvres, double & single hung, awning sashes & hoppers, sliding windows and lastly fixed glazed windows (most exp to least exp). This is great to keep in mind when choosing wondows and the amount of them in a house design.

This house, has a fair amount of windows, nothing exceptional for a house of this size, and very few appear to be a standard 'off the shelf' type. That said, it is possible to change some of them to be a standard size etc, but like I said earlier, it will effect the look and feel of the space in the house. Comes down to how much coin you are willing to spend. All the angled windows would be custom built anyway you look at, as no off-the-shelf windows exist angled like that.

Vaulted or raked ceilings are awesome! Cant top that........unfortunately, again, they are expensive, from both a labour point of view and a non-standardness situation too.
With your typical house construction with a flat ceiling, this is a trussed roof construction, and these are cheap to build and cheap to install. Sizes are given to truss manufacturers and they build them in a factory how they need to be, then pop em on a truck and send them to your house. The builder then leans them up and puts it on the walls and presto......a roof structure.

With a vaulted or framed roof, the whole thing needs to be engineered to get the correct sizes etc, then it is built on-site, taking considerable more time & labour, as well as more engineering fees. Also the sizes of the members are usually substantially bigger than a trussed frame, which adds costs.

The other extra cost compared to flat ceilings is internal labour and materials. On the high side of the ceiling, there is more wall to be built, which means more wall sheeting & paint. Also conventional cornices that you see in flat ceilings dont really work well on an angle, so the ceiling has to be square set, which is labour hungry (mind you, it looks great, even on a flat ceiling!) You can use a cornice on a raked ceiling, but it looks cheap and dodgy.

Also framed roofs are hard to insulation effectively.

Angles themselves are expensive labour wise too....lots of extra cutting of materials etc.

The type of floor construction is also a contributing factor to cost. The cheapest way, is slab on ground on a perfectly flat site (which are few and far between). Slab on ground is also the most environmentally impactable method, as land needs to excavated to dig footings etc. A elevated on post house is more 'green friendly', but more expensive (and harder to heat and cool).
The great thing with slab on ground is it stabilises the internal temperature due to the coupling with the earth, which stays pretty constant in temp. An elevated house is harder to keep warm due to heat loss through the floor, but a slab on ground doesnt lose any through the floor. Thermal design factors also contribute to this, and I wont go into it cos its a big subject

My point being that slab on flat ground is cheapest. Elevated uses lots of timber or steel members to frame up, more labour to build, more cost. A slab on ground on a sloping site can get pricey too......retaining walls etc are expensive. Plus alot of earth moving etc, so another trade to bring in.

ok, again I've rambled on about lots of things, and I will stop soon (need too, wine is kicking in a little and I need to sit back and relax! ).

Overall the plan is great, I have no comment, as well as the design, I think it is great. Rooms are nice and big (though the master bedroom is excessive, but thats my opinion cos I dont feel they need to be overly big......you dont live in your bedroom...). The kitchen to living space is great, connection to outside to deck is great......a great design! Although this design would have to be tweaked to suit the land that you buy/have, as a design must suit the land. I mean normally you get the land first, then design around the land etc. Cos you have to get things right, certain living spaces to get sun at certain times of the year and certain season of the year etc.

Anyway, to conclude (it feels like an essay! ) The cost I reckon will be min $650000 max $850000 (really ballpark..do not quote me ). The roof is doing some interesting things and would be a tough one to resolve from a built perperstive. And this is most probably going with average to slightly above average materials, appliances, fitout and finishes etc.

My final point is this......after all I have said re cost, there is nothing worse to see a great designed house space wise, but certain details have been omitted to save money, and it looks...well.....unfinished? Not cheap, just not quite right. Sometimes, when it comes to largish, impressive looking houses with greats spaces, to go that extra and do it right is the only way. I am not saying ulta expensive oven suites and ridiculously expensive slate tiles, etc etc....but decent fitout in the kitchen and bathrooms....nothing to the max....but nice stuff. Ultimately it may cost an extra 50-80G, but the results (and future resale) with be worth it.

I have always thought it you are spending a fair nugget of money on a house (and 800g is a big nugget) what is an extra 50g! Obviously what you can afford is what you can afford......but we once had a client, who got us to do this reno, and the architect got this great design for her, suited her every need totally, but when it was priced it was 20G too dear. And she would not budge of dipping into her investments etc to get it done. So she got a redesign, and built a sub-par version of her house. Sure, it was still a good house etc, but it wasnt want she set out to do when she reno'd, so really a waste of time.

anyway, thats it, I hope any of this info helps you
And if you have any queries about anything, let me know

Oh and a parting thought........is there any reason, besides from a moral stand point, that you need to purchase the drawings? I mean really, you said yourself that you are going to tweak the design and add a bedroom, (and believe me if you do it right and tweak to suit the land you have or buy, it will change a fair bit) why dont you redesign off the stuff on the web, and save the cash, and get someone to draw it up from scratch? (though I will say the cad drawings are cheap, though they would need to be amended to do the changes and that takes time and can be more annoying than a fresh drawing).
Architects do it all the time, look through magazines and get ideas and inspiration....there is probably very few true original designs around. All you actually need to do is change 10% and copyright is nullified.

anyway, enough from me!!!!! have a good night!