I would not (personally) recommend this option.

Although house design, and indeed house building is actually a fairly simple process (which is one of the reasons why Self Build is a growing market), what you “need to know” to be able to fully design and produce drawings of a suitable quality for submission for Planning and Building Regulations Approval, really limits the job to "professionals", or people with at least a reasonable level of experience. 

In some cases, people who have “already done it a few times” may be able to utilize the knowledge they have "gleaned", to create suitably professional drawings and specifications without having to bring in "professional assistance", but this is fairly rare.

If you have a reasonable level of knowledge of either “Residential or Commercial building” (you may be a tradesman or have worked in some capacity on buiding sites of one type or another), and are fairly competent, either on the drawing board, or with a CAD package, you may be tempted to “have a go”. – Possibly with the thought of saving money, or possibly just for the satisfaction of saying that you designed your new dream home yourselves. 
If you do, just bear in mind that you will need to produce not only fully “annotated” (worded) designs, but, also a full set of Building Regulations specifications which are relevant to your drawings, which are fully up to date on all the latest details and amendments. – It’s quite a daunting task!

Another thing to consider, is that if you take more than a few days preparing the drawings, you may be taking your attention and free time away from the many other parts of the project that you will need to be planning to get the project moving in the first place.

With all aspects of one of these major projects(unless you have unlimited free time, and no particular target date for completion), you need to plan your input to make the best use of your time and your own skills. – The cost of a few hundred pounds for a professional to do your drawings, which frees you up to go and negotiate discounts (of possibly hundreds, or in some cases, thousands of pounds), and get more competitive quotes for some of the major major sections of the project, might prove to be a better use of your time, and actually save you money over all.

It is good idea for you spend some before you meet up with a designer, coming up with the initial designs (in the form of “sketch drawings”) for these main reasons:

Also, on a more practical level, you will also usually save money if you can come up with the designs for your new home or extension yourselves:
Once you have chosen a designer who you want to work with, attended your first meeting and talked about your ideas, the next step (if you don’t at that stage have any ideas for the design), will usually be for your designer to go away and produce sketches of potential layouts and external appearances for you to consider. - If you can take with you, to that first meeting, some “scaled” sketches which you can then sit down and discuss, and “scribble” some minor alterations on, you may be able to “leapfrog” that whole first section of the process.

The time and moneyyou could save this way can be significant. - Possibly 2 or 3 weeks, and anything from £200 to £1000 or so, depending on the complexity of your design, and who you are using as your designer.

Taking your initial sketches and any minor amendments, your designer can then produce your full “working drawings” and prepare them for Planning and Building Regulations approval, - whilst you are free to get on with other important aspects of the planning of the project “as a whole”.

Although I suggest that, if you can, you should do the initial designs yourselves, sometimes, this is just not an option.
 
For example: If you are busy with work, family or any of 1001 “other things”, or if you simply don’t feel confident enough to be able to produce a design which will be "good enough".

If that’s the case, then you’ll need to consider taking on a “Professional Designer” from day one.

When I use the term “Professional Designer”, you may wonder why I don’t just say “Architect”?

Well, in fact although Architects are probably the most common choice for people considering taking on a major project (such as a full Self Build), there are actually a number of other options open to you.
Although we tend to think about finding “Architects” asone of the first jobs to do when starting out on a Self Build project, in fact, when we use the term, Architects, we are actually “lumping together” at least three different types of designers:

“RIBA” Registered Architects:
(RIBA stands for the Royal Institute of British Architects). – The RIBA is the main organisation associated with Architects in the UK and it has members worldwide. If you are a full member, you are allowed to use the letters “RIBA” in your company name and your advertising, to make the statement that you are a member of this high quality institution and that you intend to offer a high level of service.

Architectural Designers / Services:
These are people who can often pretty much offer most of the same services that a full RIBA Architect can offer, but who are not members, and therefore cannot use the term "RIBA" for promoting their businesses.

Draughtsmen / women:
Will often be fully capable of offering many of the services that full RIBA Architects can offer, but they are sometimes not as highly qualified as full RIBA Architects, they are not full RIBA members and they sometimes work on smaller scale projects such as extensions, conservatories. – They are well worth contacting however, if you are thinking about taking on a full "Self Build" project as many of them are quite "willing and able" to take on such a project, and may give you the best value "design package".

You will normally find that of the three options, RIBA Architects tend to be the most expensive option ( although this is not always the case). They will usually be highly qualified people with good experience in many fields of “Commercial” and “Residential” building. They may have prestigious offices and be able to take on large commercial projects as well as “one off” residential work.

Architectural practices will tend to charge a little less than full RIBA Architectural practices for the same sort of work, but often pretty much offer the same service.

Draughtsmen / women will often have a small office, or even work from home, and be sufficiently qualified and experienced to be able to prepare full working drawings for “residential development” projects in general. You will often find that they can also be the most “informal” choice of the three. – They will often also be the cheapest.

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If you opt for a “Package Build” company, or even a “Main Contractor”, you may find that they include all the design work as part of their package.

Many larger “Package Build” and “Building” companies will either have their own design team “in house”, or if not, they will work very closely with an Architectural practice”.

This may include the initial sketches as well as the full working drawings as part of their package. - They may even offer a free basic design service BEFORE you even sign up with them (they may want to to get some idea of what you are looking for in the way of "design" before they get to the stage of discussing the details of the project more fully).

If you choose this option for your project you will normally find it to be more expensive than if you take on the Planning and Management yourselves. However, it can be the least stressful route to your dream home, and it can also sometimes be the quickest.

Which one should we choose?

"Horses for courses!”
 
It’s hard for me to tell you that “You should go for one type of designer or another”, so, to try to put it into perspective:

I design my own houses, and I design houses for my clients. I am neither a “Draughtsman”, nor an “Architect”. – I have just taught myself over the years how to “knock up” a set of drawings which will be good enough to pass a Planning Application.  

I have a standard set of “Building Regs” which I have used on recent drawings, on file on the computer, so if I need to do the regs for a new set of drawings, I can basically “copy and paste” them from my existing records, then alter them to suit the new designs, and check that there are no changes to the Building regs requirements, before submitting them to the Local Authority.  

So (if I was cheap enough), there would be nothing stopping anyone from asking me to do their designs and drawings for anything up to what I would class as a “reasonably complicated design.

On that basis, if someone like me, who has just "picked it up as I have gone along", can be capable of reasonable to expect that a Draughtsman or woman, who could design an extension, should be able to prepare a set of drawings for a simple 4 bedroom / 1100 sq ft house on a flat plot, - and possibly some far more complicated designs.

Architects and Architectural Draughtsmen / women will usually be able to take on most types of residential projects, from the simplest house (or extension), up to very complicated elaborate and “structurally complicated” projects.

 Structural Warranties:

Your designer may be able to offer you to option to cover the “Structural Warranty” on your property as part of their “package”.

This will mean that, not only do they prepare the drawings, and apply for Planning and Building Regs, but they also then go onto “supervise” the whole of the building process, issuing interim “Architects Certificates” to your funder at various stages throughout the project in order for them to release you stage payments.

Some professional designers even offer to provide the “Building Regs” inspections along with the structural warranty, - which simplifies the practicalities of getting inspections done at the various stages of the build, and the issuing of the “Final Certificates”.